Fully support ABIv2 State Mutability Field

[fubuloubu]

Closes: #2042
Closes: #1931

What I did

• Changed modifying to nonpayable in external interface definitions
• Add support for payable in external interface definitions (check for value=... iff payable)
• Write tests and docs for payable usage
• Output only stateMutability in ABI output

Description for the changelog

• Changed modifying to nonpayable in external interface definitions
• Add support for payable in external interface definitions
• Deprecated ABIv1 output in favor of ABIv2

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[codecov-commenter]

Codecov Report

Merging #2054 into master will decrease coverage by 0.53%.
The diff coverage is 71.87%.

@@            Coverage Diff             @@
##           master    #2054      +/-   ##
==========================================
- Coverage   84.91%   84.37%   -0.54%     
==========================================
  Files          83       83              
  Lines        8133     8129       -4     
  Branches     1967     1966       -1     
==========================================
- Hits         6906     6859      -47     
- Misses        738      773      +35     
- Partials      489      497       +8     

Impacted Files	Coverage Δ
vyper/context/validation/local.py	87.71% <0.00%> (ø)
vyper/parser/parser.py	86.81% <ø> (ø)
vyper/context/types/function.py	83.76% <71.42%> (-1.01%)	⬇️
vyper/context/types/meta/interface.py	87.67% <100.00%> (-1.08%)	⬇️
vyper/signatures/interface.py	80.74% <100.00%> (ø)
vyper/functions/functions.py	83.71% <0.00%> (-4.45%)	⬇️
vyper/ast/nodes.py	92.72% <0.00%> (-1.62%)	⬇️

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[fubuloubu]

Note: will try refactoring out vyper/signature in favor of adding ContractFunctionType.to_abi ABI serializer, and replacing all uses of vyper/signature in vyper/parser in another PR (see https://github.com/fubuloubu/vyper/tree/refactor/abi-serializer)

[iamdefinitelyahuman]

Looks really good. I really like that the interface logic moved into from_FunctionDef, it's much easier to grok now.

[iamdefinitelyahuman]

Note to self: Once this merges, rename ContractFunctionType to ContractFunction