Merge Context.get_placeholder and Context.get_internal_variable

[iamdefinitelyahuman]

What I did

Merge Context.get_placeholder and Context.get_internal_variable

How I did it

Both of these methods served an identical purpose and get_internal_variable was only used twice in the codebase. I've merged the logic into a single function and added docstrings to make it more clear what is happening.

How to verify it

Run tests.

Cute Animal Picture

[codecov-io]

Codecov Report

Merging #2194 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2194      +/-   ##
==========================================
+ Coverage   85.45%   85.46%   +0.01%     
==========================================
  Files          83       83              
  Lines        8505     8498       -7     
  Branches     2055     2052       -3     
==========================================
- Hits         7268     7263       -5     
+ Misses        734      733       -1     
+ Partials      503      502       -1     

Impacted Files	Coverage Δ
vyper/codegen/return_.py	100.00% <100.00%> (ø)
vyper/functions/functions.py	89.33% <100.00%> (ø)
vyper/parser/context.py	95.45% <100.00%> (+1.83%)	⬆️
vyper/parser/events.py	93.66% <100.00%> (ø)
vyper/parser/expr.py	77.55% <100.00%> (ø)
vyper/parser/external_call.py	84.37% <100.00%> (ø)
...er/function_definitions/parse_internal_function.py	98.01% <100.00%> (ø)
vyper/parser/keccak256_helper.py	78.94% <100.00%> (ø)
vyper/parser/parser_utils.py	81.31% <100.00%> (ø)
vyper/parser/self_call.py	91.73% <100.00%> (ø)
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 95f97bc...c9a18eb. Read the comment docs.