modules: add go.mod and go.sum and update to modernc.org/cc

[kortschak]

This moves towards modules support for this repo. For complete support the code generation needs to be made module aware. Until that's done GO111MODULE cannot be set to on. I think the easiest way to do that will be to conditionally determine the path to the source based on whether go list -m gonum.org/v1/gonum is successful. Whether that's done in bash or in the Go code I'm not sure yet.

Please take a look.

[codecov-io]

Codecov Report

Merging #56 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #56   +/-   ##
=======================================
  Coverage   29.64%   29.64%           
=======================================
  Files           2        2           
  Lines        6382     6382           
=======================================
  Hits         1892     1892           
  Misses       4047     4047           
  Partials      443      443

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