AST Rewrite (Batch 1)

[0x19]

Complete AST rewrite...

I've added most of the functionality one AST can offer for solidity (except assembly). Some functionality still are missing totally such as extension on the overloaded declarations but that can come in later.

This is a first batch of rewrite. Next one will be related to documentation and unit testing.
After all of that, will be adding assembly support.

Could write much more but really, what's the point. This PR is a nightmare. Initially I thought it could be smaller, however I've miss calculated everything, resulting with PR that's just unreadable.

References #16

[coveralls]

Pull Request Test Coverage Report for Build 5700357010

• 3574 of 5536 (64.56%) changed or added relevant lines in 68 files are covered.
• 13 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-9.5%) to 65.393%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ast/comments.go	43	45	95.56%
ast/continue.go	36	39	92.31%
ast/builder.go	25	31	80.65%
ast/override.go	18	27	66.67%
ast/expression.go	49	59	83.05%
ast/symbols.go	6	16	37.5%
ast/inheritance.go	62	73	84.93%
ast/error.go	46	61	75.41%
ast/event.go	50	65	76.92%
ast/yul.go	40	55	72.73%

Files with Coverage Reduction	New Missed Lines	%
ast/enum.go	1	73.42%
ast/interface.go	1	65.63%
ast/builder.go	2	81.4%
ast/function.go	3	78.54%
ast/struct.go	3	61.9%
ast/using.go	3	57.69%

Totals
Change from base Build 5535503102:	-9.5%
Covered Lines:	4244
Relevant Lines:	6490

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💛 - Coveralls