Introduce interface point for RNGs

[ablaom]

Addresses #160 .

Should not be merged until:

• Refactor fit and update #161 has been merged

on which this is based.

The new build signatures are changed from build(model, n_in, n_out)/build(model, n_in, n_out, n_channels) to

build(model, rng, n_in, n_out) -> chain
build(model, rng, n_in, n_out, n_channels) -> chain

This is breaking. I had thought to keep backwards compatibility with a fallback

build(model, rng::AbstractRNG, shape...) = build(model, shape...)

but this doesn't work unless all newbuild methods have rng explicitly typed in the signature, which is annoying (otherwise there is method ambiguity).

[codecov-commenter]

Codecov Report

Merging #164 (efa210e) into dev (eb90873) will decrease coverage by 2.64%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##              dev     #164      +/-   ##
==========================================
- Coverage   91.32%   88.67%   -2.65%     
==========================================
  Files           6        8       +2     
  Lines         265      212      -53     
==========================================
- Hits          242      188      -54     
- Misses         23       24       +1     

Impacted Files	Coverage Δ
src/builders.jl	55.55% <55.55%> (ø)
src/classifier.jl	100.00% <100.00%> (ø)
src/common.jl	96.55% <100.00%> (+4.88%)	⬆️
src/core.jl	90.27% <100.00%> (+2.77%)	⬆️
src/image.jl	100.00% <100.00%> (ø)
src/regressor.jl	100.00% <100.00%> (ø)
src/types.jl	100.00% <100.00%> (ø)

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[ablaom]

Closing in favour of #166