Split apart grammar.py and segments_*.py. Introduce Parser class.

[alanmcruickshank]

This is another meaty re-org. Reason for this change is twofold:

1. We had a couple of really big source files. Especially grammar.py, which were very difficult to navigate. This PR splits up some of those larger source files into their own modules, namely sqlfluff.core.parser.segments and sqlfluff.core.parser.grammar.
2. Dependency was really unclear between some segments and some grammars. In particular the FileSegment was a bit of an anomaly in that it depended on grammars, and also that it was instantiated directly to initiate parsing.

This PR does four things:

• Refactors grammar.py into a module of seperate source files in sqlfluff.core.parser.grammar.
• Refactors segments_base.py, segments_common.py & segment_generator.py into a module of smaller seperate source files in sqlfluff.core.parser.segments.
• Moves the main content of segments_file.py into dialect_ansi.py, with the logic in the .from_raw() method (which used to be called as the standard way of parsing a lexed file) moved into a Parser class which can be called directly, much like the way that Lexer already exists. This also has the upshot of removing the need for modules outside the parser to deal with RootParseContext and so this has been removed from the public interface of this module.

[codecov]

Codecov Report

Merging #510 into master will decrease coverage by 0.02%.
The diff coverage is 92.28%.

@@            Coverage Diff             @@
##           master     #510      +/-   ##
==========================================
- Coverage   93.14%   93.12%   -0.03%     
==========================================
  Files          32       42      +10     
  Lines        4812     4855      +43     
==========================================
+ Hits         4482     4521      +39     
- Misses        330      334       +4     

Flag	Coverage Δ
#py36	93.00% <92.09%> (-0.03%)	⬇️
#py37	93.00% <92.09%> (-0.03%)	⬇️
#py38	92.95% <91.94%> (-0.03%)	⬇️
#py39	92.95% <91.94%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
src/sqlfluff/core/parser/segments/generator.py	100.00% <ø> (ø)
src/sqlfluff/core/linter.py	88.42% <72.72%> (+0.02%)	⬆️
src/sqlfluff/core/parser/grammar/greedy.py	86.58% <86.58%> (ø)
src/sqlfluff/core/parser/grammar/delimited.py	89.15% <89.15%> (ø)
src/sqlfluff/core/parser/grammar/base.py	90.73% <90.73%> (ø)
src/sqlfluff/core/parser/grammar/sequence.py	90.81% <90.81%> (ø)
src/sqlfluff/core/parser/segments/indent.py	94.59% <94.59%> (ø)
src/sqlfluff/core/parser/helpers.py	96.66% <96.66%> (ø)
src/sqlfluff/core/parser/segments/raw.py	97.14% <97.14%> (ø)
src/sqlfluff/core/parser/grammar/anyof.py	97.19% <97.19%> (ø)
... and 24 more

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