Still somewhat experimental speedup. This appears to speed up the

Still somewhat experimental speedup.  This appears to speed up the
most common interface to Tcl, the call() method, by maybe 20-25%.

The speedup code avoids the construction of a Tcl command string from
the argument list -- the Tcl argument list is immediately parsed back
by Tcl_Eval() into a list that is *guaranteed* (by Tcl_Merge()) to be
exactly the same list, so instead we look up the command info and call
the command function directly.  If the lookup fails, we fall back to
the old method (Tcl_Merge() + Tcl_Eval()) so we don't need to worry
about special cases like undefined commands or the occasional command
("after") that sets the info.proc pointer to NULL -- let TclEval()
deal with these.