A format precision specifier consisting of a dot and no number actually does nothing and has no specified meaning. Currently this is silently ignored, but it may turn into a warning or error.

See rust-lang/rust#131159 and rust-lang/rust#136638

but exhaustiveness checking is very valuable here, so allow it to be
disabled. caveats apply. read the docs in Cargo.toml.

this makes a decode+format benchmark drop from 75s to 14s...

(decode is 5s)

both from_u8 and the build function here compiled to truly trivial code:
four instructions (mov rdi, rax; cmp 0xlim, rax; jae panic; ret) in the
hot path, and constrained register choice on the caller side. inlining
these makes for a *smaller* armv7 decoder, on the order of 5kb down from
5.5kb. in the process it also gets about 45% faster (400mb/s to 560mb/s)

inlining decode_into, then, really just helps the standalone decoder
benchmark case. this moves decode throughput from 560mb/s to 724mb/s.

capstone-rs currently binds an old capstone (4.0), where capstone 5.0 is
where much of the armv8.2+ implementation was ported over from LLVM. so,
differential testing is now pointed to a capstone-rs fork pending the
merge of https://github.com/capstone-rust/capstone-rs/pull/172