Refactored arguments to Values interface

[mailund]

This interface is cleaner, I can autogenerate code for various types (and now support all basic types and slices of same), and I support more types of callbacks plus any type that implements the right interface.

Closes #10 #9 #6 #5 and #4

[codecov]

Codecov Report

Merging #11 (4ca256b) into main (2f3c600) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              main       #11    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            5         7     +2     
  Lines          310       509   +199     
==========================================
+ Hits           310       509   +199     

Impacted Files	Coverage Δ
commands.go	100.00% <100.00%> (ø)
internal/params/paramset.go	100.00% <100.00%> (ø)
internal/vals/callbacks.go	100.00% <100.00%> (ø)
internal/vals/genvals.go	100.00% <100.00%> (ø)
internal/vals/vals.go	100.00% <100.00%> (ø)
specs.go	100.00% <100.00%> (ø)

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