Use `@` syntax to escape `_` when used as an identifier.
Remove the stage1 astgen prohibition against assigning from `_`
Note: there a few stage1 bugs preventing `_` from being used as an identifier
for a local variable or function parameter; these will be fixed by stage2.
They are unlikely to arise in real C code since identifiers starting with
underscore are reserved for the implementation.
This introduces {'} to indicate escape for a single-quoted string,
and {} to indicate escape for a double quoted string.
Without this, there would be unnecessary \' inside double quoted
strings, and unnecessary \" inside single quoted strings.
Motivated by the llvm12 branch, in the new tool I am writing for
updating target CPU features.
Zig's format system is flexible enough to add custom formatters. This PR removes the new z/Z format specifiers that were added for printing Zig identifiers and replaces them with custom formatters.
