Rework the type system

[vektah]

This is a pretty large refactor of dataloaden that allows arbitrary types for keys and values.

This removes -slice and -key entirely, and replaces them with 3 mandatory positional arguments:

• name: The name to use when generating types and constructors
• key type: the key type as a string in the format (*[])(package.)TypeName
• value type: the key type as a string in the format (*[])(package.)TypeName

Unlike before with -slice and -ptr these can be nested arbitrarily deeply, for example *[][][]*github.com/my/damn.User is now valid.

for example what used to be

dataloaden -keys int -slice github.com/vektah/dataloaden/example.User	

is now

dataloaden UserSliceLoader int []github.com/vektah/dataloaden/example.User

Replaces #24
Fixes #20

[codecov]

Codecov Report

Merging #28 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #28   +/-   ##
=======================================
  Coverage   85.57%   85.57%           
=======================================
  Files           2        2           
  Lines         104      104           
=======================================
  Hits           89       89           
  Misses         15       15

Impacted Files	Coverage Δ
example/user.go	0% <ø> (ø)	⬆️
example/userloader_gen.go	94.68% <100%> (ø)	⬆️

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