External variable names that are not valid identifiers

[HertzDevil]

Funs inside a lib can have a quoted real name if that name cannot be represented as a Crystal identifier:

lib Lib
  fun foo = "llvm.ceil.f32"
end

The same cannot be done for external variables:

lib Lib
  $foo = "llvm.ceil.f32" : Int32 # Error: expecting any of these tokens: IDENT, CONST (not 'DELIMITER_START')
end

Since both are similarly mangled and Crystal doesn't have control over the mangling scheme, I think we should allow quoted strings here for external variables too.