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%

---

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

---

💛 - Coveralls

[alamb]

Looks good to me -- thanks @ayushdg

[coveralls]

Pull Request Test Coverage Report for Build 2863233291

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 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%

---

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

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 2849247119

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 25 of 27(92.59%) changed or added relevant lines in 3 files are covered.
• 530 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 85.424%

---

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

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_common.rs	19	97.1%
src/ast/mod.rs	155	77.89%
src/parser.rs	356	83.23%

Totals
Change from base Build 2848542264:	-0.02%
Covered Lines:	9512
Relevant Lines:	11135

---

💛 - Coveralls