Change lexical id generator to automate variables' normalization

Now the lexical generator returns the identifier, not just the id
name. After, when the program is completely parsed the generated
ids all are normalized calling "close_lexical".

In addition, after the lexical phase is terminated you can generate
new ids that are already "normalized".
Move the creation of lexical genid object outside the AST builder
Fix "copy and paste" error in matrix's slice assignment function
Use lexical id generator in luacode generator to avoid conflict

Previous code was broken since was using util.genid which was removed
by a previous commit.

Fix also an error that was leaking a global variables in matrix slice
assignments.
Cosmetic variables rename in luacode generator module
Add some tests to check matrix slicing assignments