Disable EQUS expansion for raw symbols (by parsing them as strings)

[Rangi42]

Fixes #1605

The goal here is to make #raw string symbols get treated like string literals/expressions instead of getting EQUS-expanded. The long-term goal is to implement that in a way compatible with removing EQUS expansion for all symbols (#905).

This approach works by just having the lexer disable EQUS expansion for all #raw symbols, so they get lexed as ID (or LABEL). Then most of the work is done in the parser. There are three kinds of things: clearly numeric expressions (42, 2 + 3, etc), clearly string expressions ("lol", STRSUB("hello", 1, 4), etc), and identifiers, whose role depends on their context and the type of the symbol corresponding to that identifier. So now, in addition to the relocexpr_no_str distinction, it needs a string_literal distinction.

This PR needs further comparison with the alternative lexer-based approach implemented in #1649.

[Rangi42]

What feels natural to me about this:

• The lexer doesn't have to depend on the runtime definition of a symbol. If it's raw and not followed by a colon, it gets lexed as a SYMBOL, and it's the parser's job to check whether that symbol is defined as a string or not.

The downsides:

• Every place where the parser can accept a numeric or string value has to do that checking. Currently that means db, dw, dl, print, and strfmt, as well as relocexprs in general.
• More lines of code than the alternative.

[ISSOtm]

I think I like this PR better than the lexer approach, as it feels more coherent from a language PoV. (In fewer words: “less lexer hack = good”).

I do feel that, while this PR is less hacky, it's also more fragile, due to all the code repetition; however, as I suggested in one of my comments, I think this could be solved by doing a bit of refactoring.

I also did not duplicate my comment from #1649 on the man page, but since that change is identical, it applies here too.

Resolved

[Rangi42]

Thanks to ISSOtm for helping settle on and refactor this approach! <3