Use quoted identifiers in wasmprinter by default

[alexcrichton]

This commit updates wasmprinter to use quoted identifiers of the form $"foo" when necessary instead of synthesizing identifiers such as $#func4<foo>. This helps produce more readable modules by default when names are synthesized since if a name is unique but otherwise has non-identifier characters then the quoted string form can be used.

While here I've additionally changed the way that non-printable characters in strings are printed to using \u{XXX} syntax instead of \NN syntax. This makes it a bit more obvious in unicode contexts that a single character is present and not multiple.

For a module I was looking at the "before" was:

;; ...
    (func $#func73<undefined_weak:thread-local_initialization_routine_for_errno> (@name "undefined_weak:thread-local initialization routine for errno") (;73;) (type 13)
;; ...
    )
    (func $_initialize (;74;) (type 13)
;; ...
    )
    (func $#func75<initialize_char_const*_> (@name "initialize(char const*)") (;75;) (type 0) (param i32) (result i32)
;; ...

and the "after" is:

;; ...
    (func $"undefined_weak:thread-local initialization routine for errno" (;73;) (type 13)
;; ...
    )
    (func $_initialize (;74;) (type 13)
;; ...
    )
    (func $"initialize(char const*)" (;75;) (type 0) (param i32) (result i32)
;; ...,

Notably $"initialize(char const*)" is much nicer to raed

[fitzgen]

🚀