add read functions for varint BE and LE

[antonilol]

(for Bitcoin's VarInt and VarUInt)

i use this library for parsing the entire blockchain (blk files) and this functionality would be great addition to the other functions

great info site: https://learnmeabitcoin.com/technical/varint

basically the first byte tells how much bytes long the numbers is or is the number himself

[normanr]

Note that protocol buffer varints are different: They store 7 bits-per-byte, and use the most-significant-bit as a continuation bit. The encoding is called VLQ, (compared to LEB128 which is the little-endian version). Bitcoin and utf8 use a "PrefixVarint".