Adds support for parameterized decimal cast

[RCHowell]

Relevant Issues

#974

Description

Today the planner only has a cast_decimal(v) function which is used in coercions but does not support parameters. This PR adds cast_decimal(v, p, s) along with using the parameters in typing judgements.

CAST(v AS DECIMAL)      -- arbitrary precision/scale decimal
CAST(v AS DECIMAL(P))   -- decimal with precision = P and scale = 0
CAST(v AS DECIMAL(P,S)) -- decimal with precision = P and scale = S

-- | AST                        | Plan
--------------------------------|----------------------
CAST(v AS DECIMAL)           -- | cast_decimal(v)
CAST(v AS DECIMAL(P))        -- | cast_decimal(v, P, 0)
CAST(v AS DECIMAL(P,S))      -- | cast_decimal(v, P, S)

Other Information

• Updated Unreleased Section in CHANGELOG: [YES/NO]
  No

• Any backward-incompatible changes? [YES/NO]
  No

• Any new external dependencies? [YES/NO]
  No

• Do your changes comply with the Contributing Guidelines
  and Code Style Guidelines? [YES/NO]
  Yes

License Information

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[johnedquinn]

Nice work. Approving knowing that we will be adding full-fledged support for parameterized types in the future.