Export Optimisers and remove params and Optimise from tests

[CarloLucibello]

Moving forward with the full deprecation of Flux.params and of the Optimise module, this removes them from tests and documentation.

After this only explicit differentiation and the Optimisers are used in the tests.

Moreover, now Flux exports the new optimisers from Optimisers.jl instead of the old Optimisers.
Should this change be considered breaking? yes
I guess it can break code for some people using implicit gradients.

Close #2413 close #1986

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.

Project coverage is 18.13%. Comparing base (7be1ca7) to head (34bbe77).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/deprecations.jl	0.00%	15 Missing ⚠️
src/layers/show.jl	0.00%	5 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (7be1ca7) and HEAD (34bbe77). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (7be1ca7)	HEAD (34bbe77)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2495       +/-   ##
===========================================
- Coverage   34.55%   18.13%   -16.43%     
===========================================
  Files          31       31               
  Lines        1904     1897        -7     
===========================================
- Hits          658      344      -314     
- Misses       1246     1553      +307     

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