feat(options): add second unsafeGet that operates on var Option[T]

[Retrocast]

There is proc get*[T](self: var Option[T]): var T, seems reasonable to also have unsafe version of it, because it is impossible to get var T from var Option[T] without raising exceptions in a "safe" (or, rather, stdlib-provided) way. Since original unsafeGet didn't have runnableExamples, mine doesn't either, but simplest test case is the same as for the aforementioned get:

var
  x = some(42)
inc(x.unsafeGet)
assert x.unsafeGet == 43

^ it works fine with my PR, but doesn't compile without it:

Error: type mismatch
Expression: inc(unsafeGet(x))
  [1] unsafeGet(x): int

Expected one of (first mismatch at [position]):
[1] proc inc[T, V: Ordinal](x: var T; y: V = 1)
  expression 'unsafeGet(x)' is immutable, not 'var'

P.S. the #get,Option[T]_2 in doc comment just links to the version of get that accepts var Option[T] instead of normal one