Optimize nonpayable checks

[iamdefinitelyahuman]

What I did

When a contract contains no payable functions, only perform a single ASSERT CALLVALUE ISZERO at the beginning of the bytecode instead of at the start of each function.

Closes #1776

How I did it

• In parser.parser::parse_tree_to_lll, prior to parsing the individual functions, check the signature of every function to see if any are payable.
• Pass the result of this check to parser.function_definitions.parse_external_function::parse_external_function, so the per-function check may be omitted.
• If required, insert the check at the start of the LLL once it has been generated.

How to verify it

Run the tests. I added some new test cases to verify that this behavior works as expected.

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2172 into master will increase coverage by 0.90%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2172      +/-   ##
==========================================
+ Coverage   84.33%   85.23%   +0.90%     
==========================================
  Files          83       83              
  Lines        8367     8370       +3     
  Branches     2019     2021       +2     
==========================================
+ Hits         7056     7134      +78     
+ Misses        808      733      -75     
  Partials      503      503              

Impacted Files	Coverage Δ
...er/function_definitions/parse_external_function.py	95.78% <100.00%> (ø)
...yper/parser/function_definitions/parse_function.py	95.83% <100.00%> (ø)
vyper/parser/parser.py	87.23% <100.00%> (+0.42%)	⬆️
vyper/parser/lll_node.py	81.99% <0.00%> (-0.95%)	⬇️
vyper/ast/nodes.py	93.84% <0.00%> (+1.53%)	⬆️
vyper/parser/expr.py	77.04% <0.00%> (+4.91%)	⬆️
vyper/functions/functions.py	89.71% <0.00%> (+5.34%)	⬆️
vyper/context/types/value/numeric.py	84.14% <0.00%> (+6.09%)	⬆️

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