Remove env_var and command_line from options (#1240)

Removed `from_env_var` and `from_command_line` from `MMTkOption` because
now all options can be set in both ways.

Replaced `Options::set_from_command_line` and
`Options::set_from_env_var` with a single `Options::set_from_string`.

Moved several methods of `Options` out of the `options!` macro if they
don't need to be generated via macro expansion. Now only two methods are
generated by the macro.

- `set_from_string_inner`: Requires matching field names against
user-provided strings.
-   `new`: Initializing fields using default values.

In order to replicate the behavior that `Options::read_env_var_settings`
silently ignores unrecognized option keys in `MMTK_*` environment
variables, we introduced an error type `SetOptionByStringError` which
`Options::set_from_string_inner` returns so that
`Options::read_env_var_settings` and `Options::set_bulk_from_string` can
behave differently when encountering invalid keys.

Author: wks

docs/userguide/src/migration/prefix.md

@@ -30,6 +30,29 @@ Notes for the mmtk-core developers:
 
 <!-- Insert new versions here -->
 
+## 0.32.0
+
+### `Options` no longer differentiates between environment variables and command line arguments.
+
+```admonish tldr
+We replaced both `Options::set_from_command_line` and `Options::set_from_env_var` with
+`Options::set_from_string` because all options can now be set via either environment variable or
+command line arguments.
+```
+
+API changes:
+
+-   module `util::options`
+    +   `Options::set_from_command_line`: Removed.
+        *   Use `Options::set_from_string` instead.
+    +   `Options::set_from_env_var`: Removed.
+        *   Use `Options::set_from_string` instead.
+    +   `Options::set_bulk_from_command_line`: Removed.
+        *   Use `Options::set_bulk_from_string` instead.
+    +   All `<T>` in `MMTKOption<T>` now must implement `FromStr`.
+        *   This means you can parse a string into `T` when setting an `MMTKOption<T>`.  For
+            example, `options.plan.set(user_input.parse()?);`.
+
 ## 0.30.0
 
 ### `live_bytes_in_last_gc` becomes a runtime option, and returns a map for live bytes in each space

src/mmtk.rs

@@ -81,13 +81,13 @@ impl MMTKBuilder {
 
     /// Set an option.
     pub fn set_option(&mut self, name: &str, val: &str) -> bool {
-        self.options.set_from_command_line(name, val)
+        self.options.set_from_string(name, val)
     }
 
     /// Set multiple options by a string. The string should be key-value pairs separated by white spaces,
     /// such as `threads=1 stress_factor=4096`.
     pub fn set_options_bulk_by_str(&mut self, options: &str) -> bool {
-        self.options.set_bulk_from_command_line(options)
+        self.options.set_bulk_from_string(options)
     }
 
     /// Custom VM layout constants. VM bindings may use this function for compressed or 39-bit heap support.