Compute and use constant expressions via the type checker.

[kalexmills]

This PR uses Go's type checker to compute and use constant expressions everywhere.

This will close #32 and will additionally support arbitrary numerical calculations when -numbers is set.

It will break compatibility with golangci-lint, as goconst will now require type information to be used. We'll need a PR upstream to plumb it through.

This PR ignores type checking errors in several locations. This is because the type checker checks a lot of extraneous information that we do not care about. The type checker can typically determine the value and type of constant expressions even if other errors are found in the file. In case type checking totally fails, treeVisitor falls back on its prior behavior.

Note: Since this PR casts numbers to strings prior to checking for collisions, we should have a lot more tests for various kinds of large numbers, and especially tests to check different floating point representations.

[codecov]

Codecov Report

Attention: Patch coverage is 74.57627% with 45 lines in your changes missing coverage. Please review.

Please upload report for BASE (master@0244082). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
parser.go	72.72%	39 Missing and 3 partials ⚠️
visitor.go	81.25%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #35   +/-   ##
=========================================
  Coverage          ?   65.66%           
=========================================
  Files             ?        4           
  Lines             ?     1031           
  Branches          ?        0           
=========================================
  Hits              ?      677           
  Misses            ?      321           
  Partials          ?       33           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[kalexmills]

@jgautheron Let's rebase this on your branch so we can keep the tests and CLI changes. I should have time to do so this evening, but feel free to do it sooner if it's blocking anything.

[kalexmills]

@jgautheron here is a combination of both our branches which ought to be able to go in without breaking anything.

[jgautheron]

Nice work, lgtm!