Decimal precision and scale enum

[AugustoFKL]

Implementation of an enum to handle Decimal (1) precision and scale, instead of options, for two reasons:

• An enum is easier to access and handle. Options need unwrap or errors even when you KNOW that an item should exist (e.g., DATETIME (p,s), if s is some, necessarily p is some).
• Remove unwrap from our code. Even if they shouldn't happen, it's not nice to have unwraps as "certainty" handlers (that was happening when we printed stuff).

Since we already didn't accept DECIMAL data type only with scale, that doesn't change the behavior of anything.

[1] : https://jakewheat.github.io/sql-overview/sql-2016-foundation-grammar.html#exact-numeric-type

[AugustoFKL]

@alamb I based this one on #648, so it duplicates its changes. Is there a better way to approach situations like that?

I use its structure, so I'm kinda stuck on that. Trying to work on things that don't conflict isn't easy hehe

[coveralls]

Pull Request Test Coverage Report for Build 3182221106

• 27 of 28 (96.43%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 85.661%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/data_type.rs	9	10	90.0%

Totals
Change from base Build 3181450573:	0.03%
Covered Lines:	10287
Relevant Lines:	12009

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💛 - Coveralls