Store Boyer Moore no case strings in lower case.

Rather than converting the search string to lower case while searching,
convert it to lowercase during initialization.

Changes the Boyer Moore search API for take BmCtx

Change the API for BoyerMoore to take a BmCtx rather than the two parts that
are stored in the context. Which is how it is mostly used. This enforces
always calling BoyerMooreCtxToNocase() to convert to no-case.

Use CtxInit and CtxDeinit functions to create and destroy the context,
even in unit tests.