Syntax of mangled names is outdated

[EFanZh]

I noticed that the syntax of mangled names specified in RFC 2603 is outdated. Specifically, it does not match the implementation of https://github.com/alexcrichton/rustc-demangle/.

Here are some differences I have observed from rustc-demangle:

1. <binder> is optional in <fn-sig> and <dyn-bounds>;
2. <const-data> may contain an optional "n" character to indicate negative values.
3. The placeholder variant of <const> does not contain a <type> item.

Also, the symbol after <fn-sig> is := while the symbol after other non-terminals are =.

[michaelwoerister]

Thanks for the bug report, @EFanZh!