Fix vector_space(K, polynomials)

[fingolfin]

The preimage function this provided was buggy; it failed when the number of monomials involved exceeded the number of polynomials. The new tests catch this.

Also add a rudimentary doc string, and refactor the code to use exponent vectors instead of "monomials".

@Lax202 that should fix the case you had trouble with

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.05882% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 80.93%. Comparing base (5d26c4c) to head (2503766).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3717      +/-   ##
==========================================
- Coverage   81.34%   80.93%   -0.41%     
==========================================
  Files         577      577              
  Lines       78611    78580      -31     
==========================================
- Hits        63945    63598     -347     
- Misses      14666    14982     +316     

Files	Coverage Δ
src/Rings/mpoly-graded.jl	90.78% <97.05%> (-0.30%)	:arrow_down:

... and 35 files with indirect coverage changes

[fingolfin]

@thofma ah I saw your name attached to the code in tig blame but that was a red herring from some global refactoring commits -- sorry! The true author of the code seems to be @fieker

[joschmitt]

I added a backport label because this constitutes a bugfix. If this gets backported, then #3727 should also be backported.