optimize float -> f64 conversions on non-abi3

[davidhewitt]

On non-abi3 builds we can use PyFloat_AS_DOUBLE to avoid an FFI function call and instead read the double value directly from the PyFloatObject structure.

This gives nearly a 2x speedup to f64 extraction when the input type is float according to the benchmark I added:

main

f64_from_pyobject       time:   [1.9698 ns 1.9870 ns 2.0047 ns]

branch

f64_from_pyobject       time:   [1.0878 ns 1.0977 ns 1.1081 ns]
                        change: [-45.172% -44.325% -43.403%] (p = 0.00 < 0.05)

Given I would expect this is the majority case of f64 conversions I think the slight bump in complexity is worth it given the drop in overhead.

While I was at it I moved the ffi definitions around to match their positioning in 3.12's headers.