Metalhead integration

[ablaom]

To close #205 and #162.

Does not need FluxML/Metalhead.jl#176 but includes a temporary hack that can be removed when that is resolved. This PR defines a generic image_builder(Constructor) wrapper, but this won't work as advertised (and cannot be made public) until FluxML/Metalhead.jl#176 is resolved.

WIP. See the image_builder docstring for more details.

Todos (incomplete):

• Add catch for disallowed kwargs in metal(Constructor)(args..., kwargs...)
• Add show method for the wrappers

[codecov-commenter]

Codecov Report

Merging #206 (64f9b3d) into dev (b3fa3a8) will decrease coverage by 3.18%.
The diff coverage is 63.33%.

@@            Coverage Diff             @@
##              dev     #206      +/-   ##
==========================================
- Coverage   92.27%   89.09%   -3.19%     
==========================================
  Files           9       10       +1     
  Lines         246      275      +29     
==========================================
+ Hits          227      245      +18     
- Misses         19       30      +11     

Impacted Files	Coverage Δ
src/builders.jl	100.00% <ø> (ø)
src/metalhead.jl	62.06% <62.06%> (ø)
src/types.jl	85.71% <100.00%> (ø)

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[ablaom]

Closed in favour of #208