Add free_associative_algebra_type

[lgoettgens]

This is analogous to dense_poly_ring_type and friends, and can be used to assert concrete types in downstream code that currently uses the abstract type FreeAssociativeAlgebra (even though we currently only have one subtype, namely Generic.FreeAssociativeAlgebra).

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.03%. Comparing base (3fe0405) to head (5abf56e).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/FreeAssociativeAlgebra.jl	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1826      +/-   ##
==========================================
- Coverage   88.03%   88.03%   -0.01%     
==========================================
  Files         119      119              
  Lines       29988    29992       +4     
==========================================
+ Hits        26399    26402       +3     
- Misses       3589     3590       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[fingolfin]

Fine by me