Refactor main compiler logic

[iamdefinitelyahuman]

What I did

• Refactor vyper/compiler.py into a subpackage
• Make the compilation process more linear
• Make the compilation process almost 5x faster! 😎

How I did it

• Compilation is handled in five phases: ast generation, context, lll gen, assembly, bytecode
• Compiler outputs trigger these phases as data is required - nothing is repeated, nothing is generated that isn't needed

Everything is well-documented in the docstrings and module README.md - see the diff for more details on how it all works.

How to verify it

Run tests.

Cute Animal Picture

[codecov-io]

Codecov Report

Merging #1962 into master will increase coverage by 0.07%.
The diff coverage is 97.10%.

@@            Coverage Diff             @@
##           master    #1962      +/-   ##
==========================================
+ Coverage   86.05%   86.13%   +0.07%     
==========================================
  Files          56       58       +2     
  Lines        6628     6641      +13     
  Branches     1685     1684       -1     
==========================================
+ Hits         5704     5720      +16     
+ Misses        585      584       -1     
+ Partials      339      337       -2     

Impacted Files	Coverage Δ
vyper/parser/parser.py	93.40% <83.33%> (-2.10%)	⬇️
vyper/compiler/phases.py	94.66% <94.66%> (ø)
vyper/compiler/utils.py	96.29% <96.29%> (ø)
vyper/ast/folding.py	80.24% <100.00%> (ø)
vyper/ast/natspec.py	98.75% <100.00%> (ø)
vyper/compiler/output.py	100.00% <100.00%> (ø)
vyper/parser/global_context.py	80.76% <100.00%> (ø)

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