Backout Unicode bare keys

This backs out the unicode bare keys from #891.

This does *not* mean we can't include it in a future 1.2 (or 1.3, or
whatever); just that right now there doesn't seem to be a clear
consensus regarding to normalisation and which characters to include.
It's already the most discussed single issue in the history of TOML.

I kind of hate doing this as it seems a step backwards; in principle I
think we *should* have this so I'm not against the idea of the feature
as such, but things seem to be at a bit of a stalemate right now, and
this will allow TOML to move forward on other fronts.

It hasn't come up *that* often; the issue (#687) wasn't filed until
2019, and has only 11 upvotes. Other than that, the issue was raised
only once before in 2015 as far as I can find (#337). I also can't
really find anyone asking for it in any of the HN threads on TOML.

Reverting this means we can go forward releasing TOML 1.1, giving people
access to the much more frequently requested relaxing of inline tables
(#516, with 122 upvotes, and has come up on HN as well) and some other
more minor things (e.g. `\e` has 12 upvotes in #715).

Basically, a lot more people are waiting for this, and all things
considered this seems a better path forward for now, unless someone
comes up with a proposal which addresses all issues (I tried and thus
far failed).

I proposed this over here a few months ago, and the responses didn't
seem too hostile to the idea:
#966 (comment)

Author: arp242

toml.abnf

@@ -50,19 +50,7 @@ key = simple-key / dotted-key
 val = string / boolean / array / inline-table / date-time / float / integer
 
 simple-key = quoted-key / unquoted-key
-
-;; Unquoted key
-
-unquoted-key = 1*unquoted-key-char
-unquoted-key-char = ALPHA / DIGIT / %x2D / %x5F         ; a-z A-Z 0-9 - _
-unquoted-key-char =/ %xB2 / %xB3 / %xB9 / %xBC-BE       ; superscript digits, fractions
-unquoted-key-char =/ %xC0-D6 / %xD8-F6 / %xF8-37D       ; non-symbol chars in Latin block
-unquoted-key-char =/ %x37F-1FFF                         ; exclude GREEK QUESTION MARK, which is basically a semi-colon
-unquoted-key-char =/ %x200C-200D / %x203F-2040          ; from General Punctuation Block, include the two tie symbols and ZWNJ, ZWJ
-unquoted-key-char =/ %x2070-218F / %x2460-24FF          ; include super-/subscripts, letterlike/numberlike forms, enclosed alphanumerics
-unquoted-key-char =/ %x2C00-2FEF / %x3001-D7FF          ; skip arrows, math, box drawing etc, skip 2FF0-3000 ideographic up/down markers and spaces
-unquoted-key-char =/ %xF900-FDCF / %xFDF0-FFFD          ; skip D800-DFFF surrogate block, E000-F8FF Private Use area, FDD0-FDEF intended for process-internal use (unicode)
-unquoted-key-char =/ %x10000-EFFFF                      ; all chars outside BMP range, excluding Private Use planes (F0000-10FFFF)
+unquoted-key = 1*( ALPHA / DIGIT / %x2D / %x5F ) ; A-Z / a-z / 0-9 / - / _
 
 ;; Quoted and dotted key
 

toml.md

@@ -106,11 +106,9 @@ first = "Tom" last = "Preston-Werner" # INVALID
 
 A key may be either bare, quoted, or dotted.
 
-**Bare keys** may contain any letter-like or number-like Unicode character from
-any Unicode script, as well as ASCII digits, dashes and underscores.
-Punctuation, spaces, arrows, box drawing and private use characters are not
-allowed. Note that bare keys are allowed to be composed of only ASCII digits,
-e.g. 1234, but are always interpreted as strings.
+**Bare keys** may only contain ASCII letters, ASCII digits, underscores, and
+dashes (`A-Za-z0-9_-`). Note that bare keys are allowed to be composed of only
+ASCII digits, e.g. `1234`, but are always interpreted as strings.
 
 ℹ️ The exact ranges of allowed code points can be found in the
 [ABNF grammar file][abnf].
@@ -120,23 +118,18 @@ key = "value"
 bare_key = "value"
 bare-key = "value"
 1234 = "value"
-Fuß = "value"
-😂 = "value"
-汉语大字典 = "value"
-辭源 = "value"
-பெண்டிரேம் = "value"
 ```
 
 **Quoted keys** follow the exact same rules as either basic strings or literal
-strings and allow you to use any Unicode character in a key name, including
-spaces. Best practice is to use bare keys except when absolutely necessary.
+strings and allow you to use a much broader set of key names. Best practice is
+to use bare keys except when absolutely necessary.
 
 ```toml
 "127.0.0.1" = "value"
 "character encoding" = "value"
+"ʎǝʞ" = "value"
+'key2' = "value"
 'quoted "value"' = "value"
-"╠═╣" = "value"
-"⋰∫∬∭⋱" = "value"
 ```
 
 A bare key must be non-empty, but an empty quoted key is allowed (though
@@ -157,7 +150,6 @@ name = "Orange"
 physical.color = "orange"
 physical.shape = "round"
 site."google.com" = true
-பெண்.டிரேம் = "we are women"
 ```
 
 In JSON land, that would give you the following structure:
@@ -171,9 +163,6 @@ In JSON land, that would give you the following structure:
   },
   "site": {
     "google.com": true
-  },
-  "பெண்": {
-    "டிரேம்": "we are women"
   }
 }
 ```