Can't parse 128-bit integer literals

[Arnavion]

As part of updating the tests/rust submodule for the catch-expressions PR, I found syn can't parse u128 literals.

let raw = "0x112233445566778899aabbccddeeff11u128";
let actual = parse_expr(raw).unwrap(); // fails

Prevents test_round_trip from parsing atleast tests/rust/src/libstd/net/ip.rs (parse_crate stops before the entire mod tests item that contains the literal, then complains that there are unparsed tokens).

Affected files:

• tests/rust/src/libcompiler_builtins/lib.rs
• tests/rust/src/libstd/net/ip.rs
• tests/rust/src/test/run-pass/i128.rs
• tests/rust/src/test/run-pass/i128-ffi.rs
• tests/rust/src/test/run-pass/issue-38987.rs
• tests/rust/src/test/run-pass/u128.rs

[dtolnay]

Thanks! We will need to figure out how to represent the parsed result. Libsyntax just puts i128/u128 in the AST but we can't do that on stable.