[cindex.py]: Speed up lookup of the completion kind

We can directly the number of the kind instead of going through the
completionChunkKindMap.

Formatting time changes from 0.84 to 0.72 seconds.
[cindex.py] Replace CachedProperty with our own implementation

This is a very performance critical point for auto completion. The manual
implementation gives a large speedup. As it does not complicate the code a lot,
I figured it is worth the change. If anybody understands why the CachedProperty
is here so much slower, I am very interested in working on an improvement of
CachedProperty.

Formatting time changes from 0.72 to 0.57 seconds.
[cindex.py] Add cache for CompletionChunk spellings

Most of the CompletionChunks represent braces, colons or other one
character spellings. There is no need to call libclang, to figure out
how to write a colon. Instead we use an internal cache to retrieve the
correct spelling. As function calls from python are very expensive and
this is a performance critical part of auto completion this patch makes
formatting of auto completion results a lot faster.

Formatting time changes from 0.57 to 0.45 seconds