feat: i16 types

[chesedo]

Required Functionality Currently Synth allows me to generate data for int4 and int8 Postgres types using the i32 and i64 generator. However, it is not possible to generate data for a int2 Postgres type.

This is because Postgres requires bytes lengths to match exactly. Hence the i32 and i64 generator cannot be used.

Proposed Solution Adding an i16 type to Synth will allow me to fill an int2 Postgres field.

Use case Filling int2 Postgres fields

[chesedo]

This should also close #271

[iamwacko]

#405 is merged