Add Generic.spoly for computing the S-polynomial of two Generic.MPoly

[fieker]

AbstractAlgebra.Generic.spoly(f, g, 2, 3) implements the spoly of f and g, assuming the leading term of f is at position 2 and for g is at position 3.
if not given, it uses the last term (position length), which should tbe the term

Mainly for @lkastner to play with the MPolyQQBar. It traverses the polynomials once and does all the exponents as inplace as possible - I hope

[codecov]

Codecov Report

❌ Patch coverage is 0% with 65 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.94%. Comparing base (41df569) to head (a605d11).
⚠️ Report is 30 commits behind head on master.

Files with missing lines	Patch %	Lines
src/generic/MPoly.jl	0.00%	65 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2347      +/-   ##
==========================================
- Coverage   88.23%   87.94%   -0.30%     
==========================================
  Files         126      126              
  Lines       32000    32173     +173     
==========================================
+ Hits        28234    28293      +59     
- Misses       3766     3880     +114     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[fingolfin]

Needs a test case or two.