MAINT: Improve speed of ufunc kwargs parsing.

Using the realization from inspecting similar CPython code that as we
intern string names anyway, comparing keys with possible names by
pointer will generally just work.

Also rather drastically rewrote, to use a new parser with simple
conversion functions to replace part of the enormous switch/case
structure, process out separately, and check for duplication after the
parsing.

Reduces the overhead as follows:
```
import numpy as np
a = np.array(1)
b = np.empty_like(a)
%timeit np.positive(a)                                 # 352->348 ns
%timeit np.positive(a, subok=True)                     # 606->501 ns
%timeit np.positive(a, where=True)                     # 586->503 ns
%timeit np.positive(a, where=True, subok=True)         # 695->531 ns
%timeit np.positive(a, b)                              # 354->352 ns
%timeit np.positive(a, out=b)                          # 557->480 ns
%timeit np.positive(a, out=b, subok=True)              # 668->506 ns
%timeit np.positive(a, out=b, where=True, subok=True)  # 752->536 ns
```

0 files changed, 0 insertions, 0 deletions