Add is_finite methods for various ring types

[fingolfin]

Also add missing (but documented) characteristic method for
FreeAssociativeAlgebra

[codecov]

Codecov Report

Attention: Patch coverage is 41.17647% with 10 lines in your changes missing coverage. Please review.

Project coverage is 88.10%. Comparing base (69ba0a5) to head (791f95f).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/LaurentPoly.jl	0.00%	2 Missing ⚠️
src/Rings.jl	60.00%	2 Missing ⚠️
src/FreeAssociativeAlgebra.jl	50.00%	1 Missing ⚠️
src/LaurentMPoly.jl	50.00%	1 Missing ⚠️
src/MPoly.jl	50.00%	1 Missing ⚠️
src/MatRing.jl	50.00%	1 Missing ⚠️
src/NCPoly.jl	0.00%	1 Missing ⚠️
src/Poly.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1842      +/-   ##
==========================================
- Coverage   88.14%   88.10%   -0.04%     
==========================================
  Files         120      120              
  Lines       30019    30031      +12     
==========================================
- Hits        26459    26458       -1     
- Misses       3560     3573      +13     

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[fingolfin]

@lgoettgens improved it. note that lots of other things are broken for (m)poly/series/... rings over zero rings, e.g. isone(one(R)) frequently returns false, because their is_one method check for the degree being 0, but since one(R) == zero(R) it isn't.

This could be fixed at the price of making is_one slower for everyone. But more problems will crop up afterwards. E.g.:

julia> R = residue_ring(ZZ, 1)[1]
Residue ring of integers modulo 1

julia> S,t = R[:t]
(Univariate polynomial ring in t over R, 0)

julia> isone(one(S))
false

julia> t == t^1
false

julia> length(t)
0

julia> length(t^1)
2

Anyway I won't resolve that here. I am still tempted to say "zero rings as coefficient rings are verboten and either outright rejected or lead to undefined behaviors" ...