Stop using TokenStream::from_str to implement syn::parse_str

[dtolnay]

The FromStr impl has bizarre hygiene behavior because it was stabilized back in Rust 1.15.0. Instead we should have syn::parse_str do something more like LitStr::parse by respanning the input to a reasonable span, probably Span::call_site().

I believe the bizarreness is tracked in rust-lang/rust#50050 -- the spans produced by FromStr are intended to be "copy and paste hygiene" but are somehow not actually.

@alexcrichton

[dtolnay]

This has been obviated upstream by rust-lang/rust#51762.