Add assertions that will guide and protect more discovery of dead code for elimination.
All use of the evalFlag value TCL_EVAL_CTX is unused by the code and
unreachable by extensions.  This checkin removes all the code supporting
that flag value.

The consequence is that all the calls to TclNREvalObjEx() and its callers
that are currently choosing not to pass the TCL_EVAL_DIRECT flag in when
they pass in a non-NULL invoker will no longer be free to change their
mind.  That might be reason not to adopt this change.
Add comment stating new limitation on Tcl(NR)EvalObjEx() interface.
Replace potentially memleak creating safety check of a "cannot happen"
condition with an assertion.
Test for TclContinuationsGet() usage, and simplifications.
Remove Tcl_Preserve support for ContLineLoc values.  It's not needed.
This allows the clLoc field of CompileEnv struct to go away too.
Many simplifications to the TIP 280 machinery.
   * Removed support for non-NULL invoker when TCL_EVAL_DIRECT requested.
   * Eliminated TCL_EVAL_CTX eval flag.
   * Removed Tcl_Preserve-ability of ContLineLoc pointers.
   * Removed clLoc field of CompileEnv struct.