Speed up reading/writing slices with #[inline]

Speed up reading/writing slices with #[inline]

When built with `rustc -O`:

before:

test bench_read_slice  ... bench:        68 ns/iter (+/- 56)
test bench_read_vec    ... bench:        78 ns/iter (+/- 21)
test bench_write_slice ... bench:       133 ns/iter (+/- 46)
test bench_write_vec   ... bench:       308 ns/iter (+/- 69)

after:

test bench_read_slice  ... bench:        32 ns/iter (+/- 10)
test bench_read_vec    ... bench:        32 ns/iter (+/- 8)
test bench_write_slice ... bench:        53 ns/iter (+/- 12)
test bench_write_vec   ... bench:       247 ns/iter (+/- 172)