implemented <$$> command for returning from a macro

 * <$$> is faster than jumping to the end of the macro
   and enables shorter code for returning values from macros.
 * this also replaces $$ as an immediate editing command.
   In other words, command line termination is an ordinary command
   now. The old behaviour was similar to what classic TECO did.
   Classic TECO however had no choice than to track key presses
   directly for command line termination as it did not keep track
   about the parser state as input was typed.
   This led to some glitches in the language. For instance
   "FS$$" would terminate the command line, unless the second escape
   was typed after backspace, etc. This behaviour is not worth copying
   and SciTECO did a better job than that by making sure that at least the
   second escape is only effective if it is not part of language syntax.
   This still lead to some undesirable cases like "ES...$$$" that would
   terminate the command line unexpectedly.
   To terminate the command line after something like "FS$$", you will
   now have to type "FS$$$$".
 * As it is a regular command now - just executed immediately - and
   its properties stay close to the macro return behaviour, command line
   termination may now not always be performed when $$ is typed even
   as a standalone command. E.g. "Ofoo$ !bar!$$ !foo!Obar$" will
   curiously terminate the command line now.
 * This also means that macros can finally terminate command lines
   by using the command line editing commands ({ and }) to insert
   $$ into the command line macro.
   This is also of interest for function key macros.
 * This implementation showed some serious shortcoming in SciTECO's
   current parser that yet have to be fixed.
   E.g. the macro "@^Ua{<$$>}" is currently unsafe since
   loops abuse the expression stack for storing their state and $$
   does not touch the expression stack. Calling "Ma>" would actually
   continue the loop jumping to the beginning of the command line
   since program counters referring to the macro A will be reused!
   This cannot be easily solved by checking for loop termination
   since being able to return that way from loops is a useful
   feature. This is a problem even without loops and $$, e.g. as
   in "@^Ua{1,2,3(4,5} Ma)".
   Instead, a kind of expression stack frame pointer must be
   added to macro invocation stack frames, pointing to the beginning
   of the expression stack for the current frame.
   At the end of macros or on return, the stack contents of
   corresponding to the frame can be discarded while preserving the
   immediate arguments at the time of the return or end-of-macro.
   This would stabilize SciTECO's macro semantics.
 * When a top-level macro returns in batch mode, it would
   be a good idea to use the last argument to calculate the
   process return code, so it can be set by SciTECO scripts (TODO).

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