Use `ubidi` as the standard name for the module.

This matches the function and constant names in the libicu source better.
Allow UBIDI_LTR/UBIDI_RTL as paraLevel.
Implement setContext/writeReordered and fix allocation issues.

We need to copy the prologue, epilogue, and main text and keep it alive
until we're done using the UBiDi object.  Add some tests to show that
everything's working now.