## gmpy_cffi is slow

Today I released version 0.1 of gmpy_cffi, a PyPy compatible implementation of gmpy. The problem is that my implementation is quite a bit slower.

As you can see, the time taken to initialize an mpz instance is about 10x slower under gmpy_cffi when compared to gmpy.

## Explanation

The gmpy mpz initialisation code uses the CPython internals to access the raw bits storing the value of a python integer (or long). In comparison, gmpy_cffi converts the number to a hex string and then uses mpz_set_str with base 16 to set the GMP mpz value

```
def _pyint_to_mpz(n, a):
"""
Set `a` from `n`.
:type n: int,long
:type a: mpz_t
"""
if -sys.maxsize - 1 <= n <= sys.maxsize:
gmp.mpz_set_si(a, n)
elif sys.maxsize < n <= MAX_UI:
gmp.mpz_set_ui(a, n)
else:
gmp.mpz_set_str(a, hex(n).rstrip('L').encode('UTF-8'), 0)
```

I've tried other methods such as using bit twiddling to break the python int into an array of C ints and then using mpz_import, but it was slower than using hex

```
def _pyint_to_mpz(n, a):
"""
Set `a` from `n`.
:type n: int,long
:type a: mpz_t
"""
neg = n < 0
n = abs(n)
tmp = array.array('L')
size = tmp.itemsize
numb = (8 * size)
mask = ~(~0 << numb)
while n:
v = n & mask
n = n >> numb
tmp.append(v)
addr, count = tmp.buffer_info()
gmp.mpz_import(a, count, -1, size, 0, 0, ffi.cast('void *', addr))
if neg:
gmp.mpz_neg(a, a)
```

## Conclusion

The conversion from python long to gmp mpz is by far the main reason why initializing mpzs in gmpy_cffi is so slow, but there isn't really an obvious way to improve on this.

Perhaps cffi could provide a fast method for converting python longs to an array of C ints, but I doubt there's a huge demand for that right now.

We're within a factor of 10 of evil low level C hackery. For now that will have to do.