Remove real-time search for routines names and more

[GillesDuvert]

1. GDL is constantly looking for values associated to names: variable names, key names, and routine names. It unfortunately does it with a naive loop on the corresponding unsorted string vector. Worse, it does it each time a routine is called, a variable is passed etc. I have modified the code so that the routine names are in an unordered set, and, when searched, the index of the name is the index of the corresponding Pro() or Fun() entry. This makes it quite faster.
2. Another point of slowness was the possibility in a C++ routine to look to a keyword using its name, not its index. This is now discouraged (Marc Schellens already warned about this a long time ago). In almost all cases, using a static int as the return of e->KeywordIx() insures that all further calls to the routine will not look again for the (same) index. All the code using non-static such ints has been reveiewed and converted to static when possible (almost always). The value for!ERR was frequently searched in the code, now a specialzed function removes this need.
3. In contrary, it is very difficult to instrument the GDL code to avoid naive O(N) name searches for keywords and variables (say: sort the name array and perform O(log(N)) binary searches), especially as the variables passed by GDL to a function are in an order that mangles keyword-relted variables and parameter variables. A partial solution has been implemented but tests have shown no improvement at all due to the overhead of adding sorted keys etc. So this has to stay as it is now.
4. In passing a number of other little improvements have been acted:

• parser optimization in primary_expr()
• test of wheter a variable in a loop may be the loop index, and warning if it is modified inside the loop was slowing the execution due to the name-to-value search discussed above: the test is removed, unless the new --warn-loop switch is set when calling gdl.
• crashes with .RESET or .RESET_SESSION are cured.
• Idem for WIDGET_CONTROL,/RESET
• operators overload and internal objects definition (overload.cpp) has been reviewed and simplified. LIST and related now are inheriting correct obejcts (CONTAINER). Undocumented (but useful) List::Set has been added.
• uninitialized elements in the plplot wxwidget driver have been set to 0, removing the annoying warnings when using Valgrind. Possibly more such warnings could be cured similarly.
• regression in message when function not found : % FCALLNode::Eval - AutoObj #2212 cured

[GillesDuvert]

victory: reference version: all tests passed on all platforms.
But we keep in mind #2200 #2177 #2176

[GillesDuvert]

successful Windows build is obtained by removing the cache.
This should be done each time a build fails for non-gdl reasons.