Variable-width integer encoding

Implement a variable-width multibyte encoding for integers in a new unittest.

Multibyte encoding using the most significant bit to indicate that the encoding continues in the following byte. Data is stored in the remaining 7 bits.

We want to be able to use a smaller amount of bits in the first byte to pack some unrelated data. For example the first byte may only use 4 data bits and the continuation bit is located on bit 5.