encourage the compiler to compile lex_keyword more efficiently

[froydnj]

I noticed while profiling on x86-64 Linux that strlen was coming up awfully high on the profile and traced it to lex_keyword:

1. There's not much reason to recompute the length of the keyword inside the function, when the caller already knows that information and has gone to some trouble to verify it prior to calling lex_keyword.
2. The version of GCC (9.4.0) I was using didn't want to inline lex_keyword without adding the inline keyword. Clang on my arm64 mac already inlines this function without inline, but this code is hot enough that I think it's worth giving an explicit hint to the compiler about what it should do.

These changes speed up parsing a subset of Stripe's codebase by 4-5%. Each change described above is responsible for about half of that speedup.