Optimize Reader.takeLeb128

Rewrite `Reader.takeLeb128` to not use `takeMultipleOf7Leb128` and
instead:
 * Use byte aligned integers
 * Turn the main reading loop into an inlined loop of static length
 * Special case small integers (<= 7 bits)

Notably signed and unsigned 32 bit integers have 5x to 12x(!)
performance improvement.

Outside of that:
For u8, u16 and u64 performance increases ~1.5x to ~6x
For i8, i16 and i64 performance increases ~1.5x to ~3.5x

For integers with bit multiples of 7 performance is roughly equal within the
margin or error.

Also expand on test coverage

Microbenchmark: https://zigbin.io/7ed5fe

Author: NicoElbers