poly-v0.4.0

• update usage in readme
• compat with clang>=11, gcc>=11
• simplify gear formula using "target" = (beta /\ f >= T)
• switch to kay::dbl::ival, add SUBDIVS param to -i, do not install SIGINT handler
• unconditionally include ival-solver
• ival-solver supports ite2
• add license headers to ival-solver
• ival-solver cleanup
• feed maybes from ival to next solver
• add simplify() on form2 and term2 + add derivative() on term2
• WIP: ival computes #critical points for simple problems
• add meson configure option -Dasserts=(true|false)
• missing case in simplify()
• add critical-points search after ival-solver