The main capi calling convention is to box all the positional
arguments into a tuple, and then pass the tuple to PyArg_ParseTuple
along with a format string that describes how to parse out the
arguments.

This ends up being pretty wasteful and misses all of the fast
argument-rearrangement that we are able to JIT out.  These unicode
functions are particularly egregious, since they use a helper
function that ends up having to dynamically generate the format
string to include the function name.

This commit is a very simple change gets some of the common cases:
in addition to the existing METH_O calling convention ('self' plus
one positional arg), add the METH_O2 and METH_O3 calling
conventions.  Plus add METH_D1/D2/D3 as additional flags that can
be or'd into the calling convention flags, which specify that there
should some number of default arguments.

This is pretty limited:
- only handles up to 3 arguments / defaults
- only handles "O" type specifiers (ie no unboxing of ints)
- only allows NULL as the default value
- doesn't give as much diagnostic info on error

The first two could be handled by passing the format string as part
of the function metadata instead of using it in the function body,
though this would mean having to add the ability to understand the
format strings.

The last two issues are tricky from an API perspective since they
would require a larger change to pass through variable-length data
structures.

So anyway, punt on those issues for now, and just use the simple
flag approach.  This cuts the function call overhead by about 4x
for the functions that it's applied to, which are some common ones:
string.count, unicode.count, unicode.startswith.
(endswith, [r]find, and [r]index should all get updated as well)