gocty type alias support

[paultyng]

Each commit is discrete, happy to open multiple PRs if you prefer (or just close this if you aren't interested).

[paultyng]

The Change interface change (and related struct changes) are breaking, so care necessary there probably.

[apparentlymart]

This diff package is, to be honest, kinda abandoned at this point. I eventually concluded it better to just retain full old/new values and produce a visual diff for humans when needed, as Terraform does.

I'm okay with making changes to it if you see a useful reason for it, and wouldn't worry at all about backward compatibility in cty/diff in particular (there should be no callers of it because it doesn't actually work) but on the other hand maybe there's enough stuff in this codebase already that this would be better off split out into its own repository and maintained separately, so that cty can get a v1.0.0 release without this experimental functionality in it.

[codecov]

Codecov Report

Merging #18 into master will increase coverage by 0.02%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   75.01%   75.04%   +0.02%     
==========================================
  Files          77       77              
  Lines        5684     5678       -6     
==========================================
- Hits         4264     4261       -3     
+ Misses       1044     1042       -2     
+ Partials      376      375       -1

Impacted Files	Coverage Δ
cty/path.go	64.7% <0%> (-4.05%)	⬇️
cty/gocty/out.go	65.18% <100%> (-0.14%)	⬇️
cty/convert/compare_types.go	97.61% <0%> (+4.76%)	⬆️

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[paultyng]

@apparentlymart just found a minor bug in gocty, I can split it out to a new PR, basically issues handling alias types (JSON does, so I looked at its implementation). I've only tested strings, now looking at other primitives.

[paultyng]

I'm fine with losing the diff functionality, the Path helpers may be useful to others though to start paths (for example, in error messages).