Rewrite `writeLeb128` to no longer use `writeMultipleOf7Leb128` and instead:
 * Make use of byte aligned ints
 * Special case small numbers (fitting inside 7 bits)

Amongst u8, u16, u32 and u64 performance gains are between ~1.5x and ~2x
Amongst i8, i16, i32 ane i64 perfromance gains are between ~2x and >4x

Additinally add test coverage for written encodings

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