Support SELECT DISTINCT, and a few minor tweaks

[nickolay]

The grammar describes this as:

    <query specification> ::=
        SELECT [ <set quantifier> ] <select list> <table expression>

    <set quantifier> ::=
        DISTINCT
      | ALL

I've chosen not to add ALL, as it's equivalent to omitting the quantifier and I have not ever seen it used in practice (I'm also afraid that some dialects might allow ALL as a column name).

(I've also included some minor unrelated changes as separate commits in this PR.)

[coveralls]

Pull Request Test Coverage Report for Build 145

• 21 of 25 (84.0%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 85.31%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/sqlparser.rs	6	7	85.71%
src/sqlast/sqltype.rs	0	3	0.0%

Totals
Change from base Build 143:	0.09%
Covered Lines:	2625
Relevant Lines:	3077

---

💛 - Coveralls

[andygrove]

LGTM!

[nickolay]

Thanks for the quick review!