Add manually implemented AST visitor

[tustvold]

Related to #78, #114, #601

This implements a basic AST visitor. I'm not convinced this is the best way to implement this, as it is relatively fragile, but I wanted to get something up to acts as a comparison point

[coveralls]

Pull Request Test Coverage Report for Build 3685169537

• 12 of 554(2.17%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-3.1%) to 83.174%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/value.rs	0	2	0.0%
src/ast/ddl.rs	0	49	0.0%
src/ast/query.rs	12	152	7.89%
src/ast/mod.rs	0	351	0.0%

Totals
Change from base Build 3641238188:	-3.1%
Covered Lines:	12541
Relevant Lines:	15078

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💛 - Coveralls

[alamb]

So what I would really like to do is to figure out some way to automatically generate this code (like using rustc_parse or something) and then automatically generate visitors (and mutators) as part of the build.rs system somehow

[alamb]

TIL: ControlFlow

[tustvold]

So what I would really like to do is to figure out some way to automatically generate this code

I'll have a play with what a proc-macro / codegen setup looks like

[tustvold]

I think the approach in #765 is much more maintainable that this, which I think would be non-trivial to maintain and would act as a potential barrier to contribution