[RPL/2] RPL/2 4.1.20

[BERTRAND Joël]

	Hello,

	New release. Some regressions introduced by new parser and new 
allocator have been fixed. All users should upgrade to 4.1.20.

Changelog :
- minor bug is fixed in evaluation(). When analyse() is called from 
evaluation(), RPL/2 always evaluates compiled expression;
- 1 -> X 'X' can be evaluated ('X' is a name and before last patch, only
algebraic expressions were able to be directly evaluated);
- major regression introduced in 4.1.19 is fixed in parser.
<< 1 -> << exp >> >> returned syntax error;
- compilation() now returns an error if a definition name contains 
invalid characters instead of BUG();
- zlib is upgraded;
- sqlite and openssl are upgraded;
- new buffercached  allocator is added;
- giac is upgraded to 1.1.4;
- Gnuplot is upgraded to 5.0.0.

	Some minors bugs and side effects remains in evaluation(). These bugs 
are triggered by DETACH and SPAWN in very rare condition (evaluation of 
a global variable in DETACH'ed RPN expression for example).

	Regards,

	JKB