new sinc, sinhc, tanc, tanhc generics + remaining trig functions #449:

• All missing trigonometric functions have been filled in sicmutils.generic
  and aliased in sicmutils.env:

  • Inverse cotangent: acot
  • inverse secant: asec
  • inverse cosecant: acsc
  • hyperbolic (inverse hyperbolic) cotangent: coth and acoth
  • hyperbolic (and inverse hyperbolic) secant: sech and asech
  • hyperbolic (and inverse hyperbolic) cosecant: csch and acsch

  All of these have default implementations and derivatives defined. They'll
  work out of the box for all types with atan defined (and potentially
  exp, sqrt and log.)

  Thanks to John D Cook's 'Bootstrapping a minimal math
  library'
  for his inspiration on the defaults and implementation order of these new
  functions.

• expt gains a new default implementation for non-native-integral powers,
  making expt work for any type with exp, log and mul defined.

• sqrt gains a default implementation for all types implementing exp,
  mul and log.

• All trig functions now have derivatives and docstrings.

• New sinc, tanc, sinhc, tanhc functions live in sicmutils.generic
  and are aliased into sicmutils.env. These are generically defined as (/ (sin x) x), (/ (tan x) x) (and similar with sinh and tanh), with
  correct definitions for 0 and infinite-valued inputs.

  These functions all support derivatives as well.

• New default acot implementation in sicmutils.series.