Implement `varuint64` and replace `varint32`

[abitmore]

varint32 is broken in bitshares-core when data is greater than 0x40000000 aka 1,073,741,824. Actually everywhere in bitshares-core is using the unsigned format unsigned_int. In addition, we're moving forward to 64 bit. Ideally the JS library should support 64 bit too.

Related issues:

• Change object_id to more than 32 bit bitshares-core#1088
• 64 bits unsigned int bitshares-fc#64
• Review and backport EOS patch about unsigned_int unpacking bitshares-core#993
• unsigned_int unpacking bitshares-core#1267