Add branch coverage reporting

[fubuloubu]

https://docs.codecov.io/docs/about-code-coverage

[codecov-io]

Codecov Report

Merging #1743 into master will decrease coverage by 4.9%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1743      +/-   ##
=========================================
- Coverage    91.9%     87%   -4.91%     
=========================================
  Files          48      48              
  Lines        5647    5647              
  Branches        0    1514    +1514     
=========================================
- Hits         5190    4913     -277     
  Misses        457     457              
- Partials        0     277     +277

Impacted Files	Coverage Δ
vyper/types/convert.py	76.66% <0%> (-11.34%)	⬇️
vyper/settings.py	80% <0%> (-10%)	⬇️
vyper/cli/vyper_json.py	84.67% <0%> (-8.88%)	⬇️
vyper/signatures/event_signature.py	83.82% <0%> (-8.83%)	⬇️
vyper/parser/global_context.py	81.4% <0%> (-7.44%)	⬇️
vyper/optimizer.py	71.42% <0%> (-7.15%)	⬇️
vyper/signatures/interface.py	85.96% <0%> (-7.02%)	⬇️
vyper/parser/memory_allocator.py	86.66% <0%> (-6.67%)	⬇️
vyper/exceptions.py	90.16% <0%> (-6.56%)	⬇️
vyper/parser/lll_node.py	84% <0%> (-6.5%)	⬇️
... and 25 more

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[fubuloubu]

Note: the large change in coverage is due to this PR enabling branch coverage, which is a stricter form of coverage many projects opt not to do. Also, their algorithm for counting branch partials is a little pessimistic IMO, but what can you do?

[charles-cooper]

Seems pretty good, are there any downsides to this

[fubuloubu]

No downsides, just looks a little worse for us because branch coverage is more strict than line coverage. To give an example:

if condition:  # Branch coverage cares about both states of this condition
    do_something()  # Line coverage only cares that this was reached
# else - Line coverage doesn't care if the false-y scenario was executed

[fubuloubu]

@jacqueswww 👍 and can merge