Performance tweaks

[3Hren]

Currently there are performance issues due to indirection between decoding a marker and a value itself; and non-inlining in the core RMP crate. Need to write benchmarks and fix them. The one approach is to mark some functions as [inline] or [inline(always)]. This works, but I feel it a little bit hacky. The other one - is to match for marker bytes directly and avoid using read_marker, but we lose an abstraction then.

[arthurprs]

IMHO #[inline] is not hacky, the attribute is essentially mandatory to allow inlining across crates.