Unsafe funcs for FreeAssAlgebra, Poly, MPoly

[lgoettgens]

cc @fingolfin

Resolves #1819

[codecov]

Codecov Report

Attention: Patch coverage is 99.33775% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.15%. Comparing base (9ebc781) to head (1001697).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/generic/FreeAssociativeAlgebra.jl	99.06%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1847      +/-   ##
==========================================
+ Coverage   88.06%   88.15%   +0.09%     
==========================================
  Files         120      120              
  Lines       30057    30203     +146     
==========================================
+ Hits        26469    26625     +156     
+ Misses       3588     3578      -10     

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[lgoettgens]

Rebasing this on top of #1838 somehow broke neg! for universal polynomial rings. I have no idea why. Can you look into this @fingolfin ?