Don't use int for bits field in gzip_index_point

[rdpsin]

Is your feature request related to a problem? Please describe.
Currently, the bits field in gzip_index_point has the type int which is 4 bytes on most implementations. However, it only needs to contain values for 0-7. The problem is compounded because we only serialize/deserialize 1 byte out of the 4. Changing it to use a smaller sized integer which shave off a few bytes, but more importantly, help get rid of pesky bugs like #24.

Describe the solution you'd like
Use something like uint8_t or int8_t to represent bits.

Describe alternatives you've considered
N/A

Additional context
N/A