Fix default params

[iamdefinitelyahuman]

What I did

Properly handle default arguments with a size > 1 word. Fixes #1987

How I did it

In vyper/parser/function_definitions/parse_external_function.py::parse_external_function, use get_size_of_type to determine the size in memory for each default argument.

How to verify it

Run the tests. I added some cases based on the example in the issue. I also verified the behavior for internal functions.

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2077 into master will decrease coverage by 0.92%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2077      +/-   ##
==========================================
- Coverage   85.04%   84.12%   -0.93%     
==========================================
  Files          83       83              
  Lines        8239     8240       +1     
  Branches     1985     1985              
==========================================
- Hits         7007     6932      -75     
- Misses        735      811      +76     
  Partials      497      497              

Impacted Files	Coverage Δ
...er/function_definitions/parse_external_function.py	95.78% <100.00%> (+0.04%)	⬆️
vyper/context/types/value/numeric.py	78.04% <0.00%> (-6.10%)	⬇️
vyper/functions/functions.py	84.37% <0.00%> (-5.35%)	⬇️
vyper/parser/expr.py	71.66% <0.00%> (-5.00%)	⬇️
vyper/ast/nodes.py	92.72% <0.00%> (-1.62%)	⬇️
...er/function_definitions/parse_internal_function.py	98.01% <0.00%> (+0.99%)	⬆️

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