encode/decode number as bitcoin variable length integer

This is a fork of the original varuint-bitcoin library with minor changes:
- codebase updated to use UInt8Array instead of Buffer - thus making this library browser-compatible without any shims
- package type changed to ESModule instead of CommonJS
- updated tests to reflect these changes




encode/decode number as bitcoin variable length integer
| value | storage length (bytes) |
|:------:|:--------------:|
| [0, 0xfd) | 1 |
| [0xfd, 0xffff] | 3 |
| [0x010000, 0xffffffff] | 5 |
| [0x0100000000, 0x1fffffffffffff] | 9 |
MIT