Missing eraser function

[Hadmut Danisch]

Hi,

this is not exactly a bug, but rather a missing function.
I'd like to use gmp for some cryptographical computations,
but could not find an erase function.

For security applications it would be *very* important to
be able to erase every single bit of information that could
leak out. So there should be function calls which allow to
erase numbers and all internal variables in gmp used for
computations.

regards
Hadmut