Remove #undef NDEBUG.
Stop doing a dynamic cast in is_modifiable( ).
Try to save some time in find_parameter_modifiable_reg( ).
Remove dynamic casting from get_trimmed( ).
Add notes about speedups.
Add code to release identifiers.
Release the identifiers if we aren't in debug mode.
Throw away identifiers before MCMC.

This means we no longer have to copy a map<string,root_t> every
time we split a context!