Calculate array index offsets at compile time

[iamdefinitelyahuman]

What I did

When accessing an array in memory, calculate the offset during compile-time instead of runtime.

How I did it

In parser_utils, I added a private method _make_array_index_setter that generates different LLL if the target value is in memory and the offset is fixed. I also avoid the use of with where possible, so no values are needlessly placed on the stack.

How to verify it

Run the tests. I didn't add any new cases, but this behavior is well tested within the existing suite.

Cute Animal Picture

[codecov-io]

Codecov Report

Merging #2187 into master will decrease coverage by 0.79%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2187      +/-   ##
==========================================
- Coverage   85.36%   84.57%   -0.80%     
==========================================
  Files          79       83       +4     
  Lines        8250     8446     +196     
  Branches     2010     2040      +30     
==========================================
+ Hits         7043     7143     +100     
- Misses        709      802      +93     
- Partials      498      501       +3     

Impacted Files	Coverage Δ
vyper/parser/parser_utils.py	81.31% <100.00%> (+0.76%)	⬆️
vyper/context/types/value/numeric.py	78.04% <0.00%> (-6.10%)	⬇️
vyper/functions/functions.py	84.40% <0.00%> (-5.34%)	⬇️
vyper/parser/expr.py	72.74% <0.00%> (-4.81%)	⬇️
vyper/ast/nodes.py	92.30% <0.00%> (-1.54%)	⬇️
vyper/opcodes.py	100.00% <0.00%> (ø)
vyper/exceptions.py	93.51% <0.00%> (ø)
vyper/__main__.py	0.00% <0.00%> (ø)
vyper/typing.py	100.00% <0.00%> (ø)

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