Change decimal bounds

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

change decimal bounds

change from (-2**127 * 10**10, 2**127-1 * 10**10) to (-2**168, 2**168).
this way we use the entire range of the underlying int, and clamps are
more efficient and easier to reason about since we can use the same
clamps as integers.

Description for the changelog

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[codecov-commenter]

Codecov Report

Merging #2730 (66fe490) into master (dfbca3e) will decrease coverage by 0.02%.
The diff coverage is 97.46%.

@@            Coverage Diff             @@
##           master    #2730      +/-   ##
==========================================
- Coverage   87.25%   87.23%   -0.03%     
==========================================
  Files          93       93              
  Lines        9896     9872      -24     
  Branches     2470     2465       -5     
==========================================
- Hits         8635     8612      -23     
- Misses        777      779       +2     
+ Partials      484      481       -3     

Impacted Files	Coverage Δ
vyper/utils.py	88.23% <89.47%> (+0.39%)	⬆️
vyper/ast/folding.py	91.73% <100.00%> (ø)
vyper/ast/nodes.py	94.13% <100.00%> (ø)
vyper/builtin_functions/convert.py	87.23% <100.00%> (+0.39%)	⬆️
vyper/codegen/core.py	84.18% <100.00%> (+0.11%)	⬆️
vyper/codegen/expr.py	81.08% <100.00%> (+0.66%)	⬆️
vyper/codegen/module.py	89.89% <100.00%> (-0.67%)	⬇️
vyper/codegen/types/types.py	87.94% <100.00%> (+1.20%)	⬆️
vyper/ir/optimizer.py	83.33% <100.00%> (-6.29%)	⬇️
vyper/semantics/types/value/numeric.py	85.05% <100.00%> (ø)
... and 3 more

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[lgtm-com]

This pull request introduces 4 alerts when merging c05deae into ce6c5d7 - view on LGTM.com

new alerts:

• 4 for Unused import