Stop injecting coverage hash fn definition in interfaces

[cgewecke]

Instrumenting interfaces triggers an error like this:

,/contracts/standards/ERC900.sol:6:1: TypeError: Functions in interfaces cannot have an implementation.
function coverage_0xdcb93d1c(bytes32 c__0xdcb93d1c) public pure {}
^----------------------------------------------------------------^
,/contracts/standards/ERC900.sol:6:1: TypeError: Functions in interfaces must be declared external.
function coverage_0xdcb93d1c(bytes32 c__0xdcb93d1c) public pure {}
^----------------------------------------------------------------^

Also adds interface test

[codecov-io]

Codecov Report

Merging #383 into truffle-plugin will increase coverage by 0.01%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##           truffle-plugin     #383      +/-   ##
==================================================
+ Coverage           94.33%   94.34%   +0.01%     
==================================================
  Files                   9        9              
  Lines                 459      460       +1     
==================================================
+ Hits                  433      434       +1     
  Misses                 26       26

Impacted Files	Coverage Δ
lib/parse.js	86.9% <100%> (+0.15%)	⬆️

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