Add sha256sum template function

[tianon]

This is intentionally named and implemented to match the one in Sprig: https://masterminds.github.io/sprig/crypto.html

I need this so that docker-library/meta-scripts#16 can calculate sourceIds in pure bashbrew formatting (and thus be really fast): 😅

...
	{{- $sum := $.ArchGitChecksum $a . -}}
	{{- $file := .ArchFile $a -}}
	{{- $builder := .ArchBuilder $a -}}
	{
		"sourceId": {{ join "\n" $sum $file $builder "" | sha256sum | json }},
		"reproducibleGitChecksum": {{ $sum | json }},
...

[yosifkit]

A test for this?

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (0b38175) 75.21% compared to head (1a4c6e8) 74.89%.

Files	Patch %	Lines
pkg/templatelib/lib.go	0.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
- Coverage   75.21%   74.89%   -0.33%     
==========================================
  Files           8        8              
  Lines         920      924       +4     
==========================================
  Hits          692      692              
- Misses        173      177       +4     
  Partials       55       55              

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[tianon]

Gah, forgot to refresh.

Edit: #91