Add binary size bounds checking

[coilysiren]

What type of PR is this?

• bug
• cleanup
• documentation
• feature
• maintenance / operations

What this PR does / why we need it:

This PR adds binary size bounds checking, so that we can avoid more people having issues like that one described in #1055.

Which issue(s) this PR fixes:

Fixes #1057

Changes

• moved build.go to internal/build.go
• I didn't change any of the existing bits of build.go 😝 (except the top comment) the git diff is just betraying me
• adds a build script for checking binary size

Special notes for your reviewer:

Here's parts of this PR that I find interesting:

• the binary size diffs are different on ubuntu / mac / windows, and different on different go versions. as of a8a1ef0 they're 2.0, 2.1.
• the specific values for the min and max sizes are interesting to me, I think there being 0.5MB between them is a comfortable distance. I would be open to decreasing the min and max to fit the current values more tightly (eg. min 4.7 max 4.9), though I changed the min and max to just be the min and max size diffs on the various platforms
• I assume that the next person to trigger test failures based on binary size won't be me, eg. it'll be some new open source contributor working on their 1st PR. For that person, do you think the included guidance is sufficient?

Potential Future Work

I thought about adding either of these things:

• https://github.com/jondot/goweight
• https://github.com/google/bloaty

I think they'd be cool to add ✨ but I'm not that invested in adding them personally.

Trivia

People really care about binary sizes! golang/go#6853

Testing

I briefly changed the min and max values to make sure that they are actually exiting with a failure code and displaying the guidance properly.

Release Notes

There's no user-facing changes in this PR ^_^

[codecov]

Codecov Report

Merging #1079 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1079      +/-   ##
==========================================
+ Coverage   72.83%   72.91%   +0.07%     
==========================================
  Files          33       33              
  Lines        2474     2481       +7     
==========================================
+ Hits         1802     1809       +7     
  Misses        565      565              
  Partials      107      107              

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[coilysiren]

Possible future optimization: instead of the raw size, use the diff between main.go with "hello world" and a main.go with urfave/cli