Fix escaping of names in IR (#8880)

The binary reader (but not the text parser) would previous escape names
read from the names section and use those escaped names in the IR. But
there is no reason to escape names in the IR; we can use the unmodified
byte sequences instead. Remove this unnecessary escaping and always
store names unescaped in the IR.

The binary writer was partially unescaping names when writing them back
to the names section, but this is no longer necessary. After removing
this, we simply preserve whatever names were present in the input,
modulo any changes we might have made to avoid duplicate names.

Asyncify and wasm-split read function names as input from the command
line and from manifest files and would previously escape these names on
reading so they would match up with the escaped names in the IR. Now
that the IR does not store escaped names, fix Asyncify and wasm-split to
no longer escape these names. Remove wasm-split's --unescape option now
that function names will always be printed unescaped.

As a follow-on change we might consider _unescaping_ names taken on the
command line or manifest files, which would allow users to more easily
pass names containing unprintable characters.