Create VarInt reader

[pablitovicente]

Description

Create function varint(buffer, schema) => value

Motivation

lisk-codec will include a set of Readers for each data type defined in the LIP, to that end
we need a function to read VarInts.

Acceptance Criteria

• varint() should accept a buffer representation of a varint value
• varint() returns either a number or a BigInt (this is controlled by the schema)
• Code should be optimised for speed as much as possible
• Full test coverage
• Performance stats are provided based on the reading of values of different sizes and measure the reading of 100, 500, 1000, 30000 varints

Additional Information

Check if it will make sense to implement signed varints in same issue.

Related issues: #5200