refactor[parser]: remove ASTTokens

[charles-cooper]

What I did

remove asttokens from the parse machinery, since the method is buggy (see below bugs) and slow (#4272). brings down parse time (time spent in ast generation) between 40-70%.

fixes multiple tokenizer-related bugs, including

• bug: wrong column offset in error message #3430
• cannot log event which contains Attribute expr #3059
• 'IndexError: list index out of range' raised calling vyper.compiler.compile_code with 54 double quotes #2258
• compiler crash due to ASTTokens instantiation #4139
• feat[ux]: improve hint for events kwarg upgrade #4275 (comment)
• feat[lang]: auto-export events in ABI #3808 (comment)

How I did it

another way to adjust tokens

How to verify it

Commit message

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[codecov]

Codecov Report

Attention: Patch coverage is 89.02439% with 9 lines in your changes missing coverage. Please review.

Project coverage is 45.01%. Comparing base (4507d2a) to head (042021c).

Files with missing lines	Patch %	Lines
vyper/ast/parse.py	91.52%	4 Missing and 1 partial ⚠️
vyper/ast/pre_parser.py	82.60%	4 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (4507d2a) and HEAD (042021c). Click for more details.

HEAD has 179 uploads less than BASE

Flag	BASE (4507d2a)	HEAD (042021c)
	180	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4364       +/-   ##
===========================================
- Coverage   91.94%   45.01%   -46.93%     
===========================================
  Files         119      119               
  Lines       16677    16716       +39     
  Branches     2807     2810        +3     
===========================================
- Hits        15333     7525     -7808     
- Misses        924     8627     +7703     
- Partials      420      564      +144     

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