Rename precision-related functions and add them to Float32 and Float64

Now get_precision is defined for all T <: FloatingPoint, and returns
24 for Float32 and 53 for Float64 (the effective number of bits in the
mantissa, taking in account the implicit leading bit.

get_bigfloat_precision and set_bigfloat_precision are used to get and set
the defaults for all BigFloats, and the BigComplex case is analogous.
Update MPCComplex to use a precise mpc_struct

As using two mpfr_structs doesn't work, I decided to expand them
directly into two groups of four elements (two longs, an int and
a pointer). They could be useful if we ever need to implement
efficient in-place algorithms, as they can be directly extracted
and manipulated as MPFRFloats.
Flexibilize the ROUNDING_MODE for MPCComplex numbers
Convert methods for other common complex types available

These methods were written in order to avoid having to use real()
or imag(), as they create a new MPFRFloat object.