replace InCB_Extend_table search with simple predicates

[cometkim]

Context: #177

(I wrote it without using the strategy of folding InCB=Consonant into the cats enum to make changes small and isolated)

---

According to UCD, the property InCB=Extend is defined as:

\p{InCB = Extend} ≔ [
\p{gcb=Extend}
\p{gcb=ZWJ}
- \p{InCB=Linker}
- \p{InCB=Consonant}
- [\u200C]
]

And no InCB=Consonant is gcb=Extend or gcb=ZWJ. So in can be determined with the grapheme category the caller already has and two equality.

scripts/unicode.py now checks that derivation against the UCD. A future Unicode version that breaks either fact fails the generator instead of silently mis-segmenting Indic text.

Also one fact that is InCB=Linker and InCB=Extend are subset of gcb=Extend, any other category rules out both without inspecting the codepoint.

This change improves the overall performance of the grapheme APIs by skipping unnecessary category checks and table searches in the hot loop. On my machine, it measures approximately -15 to -20%.

[Manishearth]

Great, thanks! These properties do change often, but the python code's preemptive check is good and also pretty clear about how to change things.

[cometkim]

Thanks for the review! next up will be also similar approach and size.