Support TRIM from with optional expr and FROM expr

[ayushdg]

Fixes #568
The objective of this pr is to allow specifying the chars from expr syntax for the trim expression even without specifying a trim where flag.

• Update trim expr members, not tying the trimWhere option to the trim_what option
• Update parsing logic to handle cases with no trim flag but trim_char
• Add tests

[coveralls]

Pull Request Test Coverage Report for Build 2863233291

• 20 of 22 (90.91%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 85.433%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	7	9	77.78%

Files with Coverage Reduction	New Missed Lines	%
src/parser.rs	1	83.25%

Totals
Change from base Build 2848542264:	-0.01%
Covered Lines:	9507
Relevant Lines:	11128

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💛 - Coveralls

[alamb]

Looks good to me -- thanks @ayushdg