Leb128 is a variable-length encoding for unsigned integers that is used
in a number of contexts - in particular, wasm, dwarf and protobuf.
This is an optimized low-level implementation that unrolls the loop
reading/writing the buffer - it is suitable to use as base for a more
specific API - no memory allocations, no exceptions.
This PR also fixes bitops2 to not raise on certaing uint->int
conversions, adapting bitops to nim 1.0 conversion rules by using a cast
instead of raising on uint->int conversion
Jacek Sieka