Implement the numeric type

[chotchki]

Postgres has arbitrary precision types that are critical for financial applications.

Implementing them will require a picking up a rust arbitrary precision library such as https://www.postgresql.org/docs/current/datatype-numeric.html

Right now the most reasonable library seems to have major limitations https://github.com/alkis/decimal and is basically a wrapper of the C library http://speleotrove.com/decimal/.

[chotchki]

This library looks far better for doing the decimal work: https://github.com/paupino/rust-decimal