v0.7.3: Type-stable mutating functions (#162)

* Type-stability fixes in ^

* Use iszero

* Parameterize mutating functions

* Fixes to few arithmetic mutable functions

* Add more tests

* Use sqr! in abs2!

* Add more tests

* Add findlast for Taylor1, with tests.

* Fixes in a^n for a::Taylor1 and integer powers

* Avoid computations in evaluate if coeff is zero

* Add REQUIRE for documentation

* Fixes in docs