Ensure sequential memory for internal variables

[iamdefinitelyahuman]

What I did

Ensure that multiple internal variable placeholders are ordered sequentially. This fixes an error in events and revert strings that was introduced by #2188

How I did it

• Add Context.new_sequential_vars, which generates multiple sequential memory offsets.
• Use the new method in parser/stmt where sequential memory offsets are required (events, revert reasons, returning bytes arrays).

How to verify it

Run tests.

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[codecov-io]

Codecov Report

Merging #2196 into master will decrease coverage by 1.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2196      +/-   ##
==========================================
- Coverage   85.46%   84.37%   -1.09%     
==========================================
  Files          83       83              
  Lines        8498     8437      -61     
  Branches     2052     2052              
==========================================
- Hits         7263     7119     -144     
- Misses        733      817      +84     
+ Partials      502      501       -1     

Impacted Files	Coverage Δ
vyper/parser/context.py	95.78% <100.00%> (+0.33%)	⬆️
vyper/parser/events.py	93.47% <100.00%> (-0.19%)	⬇️
vyper/parser/stmt.py	80.49% <100.00%> (-0.14%)	⬇️
vyper/ast/annotation.py	87.09% <0.00%> (-7.26%)	⬇️
vyper/context/types/value/numeric.py	77.50% <0.00%> (-6.65%)	⬇️
vyper/functions/functions.py	83.86% <0.00%> (-5.48%)	⬇️
vyper/parser/expr.py	72.52% <0.00%> (-5.03%)	⬇️
vyper/ast/nodes.py	91.86% <0.00%> (-1.61%)	⬇️
vyper/functions/convert.py	73.64% <0.00%> (-1.36%)	⬇️
vyper/context/types/indexable/sequence.py	82.14% <0.00%> (-0.62%)	⬇️
... and 11 more

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