mp_integer: first step towards changing the overflow signalling mechanism from exceptions
to function return value.

Tests still need to be ported. Initial performance tests indicate gains up to 10%.
mp_integer: minor comment fix.
Merge branch 'master' into new_overflow_detect
mp_integer: fix tests for the new overflow handling.
mp_integer: drop a couple of todo items.
mp_integer: replace boost integer traits with std::numeric_limits.
A couple of tweaks to the build system.

* Clarify in config.hpp which parts are generated by the configure process.
* Include a library header in the compiler version checkers, so we can catch
  earlier if, e.g., the standard library is not usable (for instance when using
  libcxx with clang on some platforms without additional includes) before
  reaching the MPFR version check (which gives a confusing error message).
testing: add a new test that forces dynamic storage for mp_integer types on 64-bit archs.
mp_integer: bits of code for the new int promotion. Not functional or tested yet.
type_traits: replace boost integer_traits with std::numeric_limits.
type_trait: add small helper that determines a symmetric range for a signed integer type
for which it is safe to take +/-.
mp_integer: more work and cleanup on the new approach to int promotion.

Also restored mp_integer<> as coefficient type in the gastineau tests.
mp_integer: tweaks, checks and tests for the static view class.
piranha.hpp: add a couple of todo items.
First uses of inherited constructors, in monomial and poisson series.

This gets rid of the use of the forwarding ctor macro and a bunch of defaulted
ctors/assignment ops as well.
poisson/polynomial term: inherited constructors.
memory: remove dead code from the test.
mp_integer: in the static view, add default ctors for later use and make sure everything
is initialised in order to avoid accessing variables with indeterminate values (this
should even be undefined behaviour in C++14).
mp_integer: mpz_view class, plus some test additions.
mp_integer: minor doc bit.