Refactor int128 clamps

[iamdefinitelyahuman]

What I did

Refactor the process of clamping int128 using bitshifting.

How I did it

The new check can be expressed as:

assert abs(n) >> 127 == 0

This check produces smaller bytecode and uses less gas at runtime.

The logic is handled in a new int128_clamp function within vyper.parser.arg_clamps. All the locations where clamping previously happened are now calling to this function.

How to verify it

Run the tests.

I still need to verify how well tested this behavior is.

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2179 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2179      +/-   ##
==========================================
+ Coverage   85.32%   85.34%   +0.01%     
==========================================
  Files          83       83              
  Lines        8403     8411       +8     
  Branches     2032     2033       +1     
==========================================
+ Hits         7170     7178       +8     
  Misses        731      731              
  Partials      502      502              

Impacted Files	Coverage Δ
vyper/functions/convert.py	75.00% <100.00%> (+0.16%)	⬆️
vyper/functions/functions.py	89.73% <100.00%> (+0.01%)	⬆️
vyper/parser/arg_clamps.py	100.00% <100.00%> (ø)
vyper/parser/expr.py	77.09% <100.00%> (+0.04%)	⬆️
vyper/parser/parser_utils.py	80.70% <100.00%> (+0.05%)	⬆️

---

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[codecov-io]

Codecov Report

Merging #2179 into master will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2179      +/-   ##
==========================================
- Coverage   85.32%   85.24%   -0.09%     
==========================================
  Files          83       79       -4     
  Lines        8403     8227     -176     
  Branches     2032     2006      -26     
==========================================
- Hits         7170     7013     -157     
+ Misses        731      714      -17     
+ Partials      502      500       -2     

Impacted Files	Coverage Δ
vyper/functions/convert.py	75.00% <100.00%> (+0.16%)	⬆️
vyper/functions/functions.py	89.73% <100.00%> (+0.01%)	⬆️
vyper/parser/arg_clamps.py	100.00% <100.00%> (ø)
vyper/parser/expr.py	77.09% <100.00%> (+0.04%)	⬆️
vyper/parser/parser_utils.py	80.54% <100.00%> (-0.11%)	⬇️
vyper/exceptions.py
vyper/opcodes.py
vyper/__main__.py
vyper/typing.py

---

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[fubuloubu]

lgtm