Flexible Varint/VLQ: Variable-length bytewise integer encodings in Go

And the decoder:

https://bugfix-66.com/1efa93a5eb0cc12b3de7cd1dab8e471a2cc95e...

This is a generalization (and improvement on!) the typical "high-bit" varint encoding, which is simply an specialization with base 128 (used in git, for example).

See https://en.m.wikipedia.org/wiki/Variable-length_quantity