Optimise Decimal binary dumper

[dvarrazzo]

There are now dumper/loader to adapt Python Decimal to Postgres numeric in binary format, implemented both in C and in Cython.

These methods however are about the 30% slower than the text format adapters (at least the dumper). The problem is that, even in C, the binary dumper uses the Decimal.as_tuple() method, which is relatively slow.

An API to access the Decimal C values should be available in Python 3.10: https://bugs.python.org/issue41324 The C adapters should use it.

There is a draft of a branch where I was thinking to use the mpdec directly, but this would complicate the build at it requires the libmpdec-dev package. So it's probably a bad idea.