752 custom units

[jacqueswww]

- What I did

Implements #752, custom units.

- How I did it

Added support for units as a custom global keyword.

# specify units used in the contract.
units: {
    cm: "centimeter",
    km: "kilometer"
}

# global storage
a: int128(cm)


@public
def test() -> int128(km):
    b: int128(km)
    b = 100
    return b

- How to verify it

See tests and exceptions in parser.py

- Description for the changelog

- Cute Animal Picture

[fubuloubu]

Later on down the line, I wonder if we could define conversion tables for units like this, where a numeric conversion exists between different units (e.g. ether)