[solvers] implement minsolve_linear_system.
[simplify] Improvements to ratsimpmodprime.

Namely:

- fix a bug where denominators would be dropped
- add a parameter quick, and use solve(.., minimal=True, quick=True)
- if quick is not True, collect all solutions of the minimal total degree (but
  differing individual degrees) and use minsolve_linear(quick=False) to find
  the best
- make staircase() much more efficient
[functions/elementary] Implement cosh(x+y) sinh(x+y) expansion.
[polys] Fix eject().

Note that the changes in densebasic are most easily tested through the
polytools wrapper classes.
[simplify] Start trigsimp_groebner.

This is a function which simplifies trigonometric and hyperbolic fraction using
the groebner basis based algorithm ratsimpmodprime.
[simplify/trigsimp_groebner] Implement polynomial simplification.

This is a faster, but less good version. Also speed up some tests.
Better integration of trigsimp_groebner with rest of trigsimp.
[simplify] Implement "deep" for trigsimp with groebner.
Implement known values of Si, Ci, Shi, Chi at oo and -oo
Tests for values of Si, Ci, Shi, Chi at oo and -oo
Replace 0 by S.Zero
add chain branching strategy
add chain to imports
factor debug to onaction and write
onaction imports
add sfilter strategy
add subfactorial function
tweak docstring eqns
add nC and nP to combinatorial numbers
new _gen_poly algorithm