build system: the Boost.System library is actually required only when building the perf tests (for the timers).
serialization: initial scaffolding.
real: implement serialization and add getters for the internal mpfr_t instance.
mp_integer: modify the copy/move assignment operators so that after the assignment the object is dynamic or static according to the assignment target.
build system: add an advanced option to signal we intend to run the code in Valgrind.

At the moment this is used only to disable some tests involving long double.
real: a minor style change in the serialization test.
mp_integer: implement serialization.
Some internal doc updates.
mp_rational: support for serialization.
real: some doc updates.
small_vector: implement serialization.
real: a very minor test addition.
sphinx: doc updates, further structure.
array_key: serialization support.
array_key: move serialization method at the top.
monomial: implement serialization.
small_vector: a small addition to the serialization test (check that the type of storage is consistent).
km: implement serialization.
rtkm: implement serialization.
symbol: implement serialization.