Fix problem with empty function's body

Fix issue:

https://github.com/franko/luajit-lang-toolkit/issues/2
Factor out the operator priority table from the parser
Factor out code to evaluate arithmetic constant AST node
Preliminary implementation of string source code backend generator

Currently work in progress. It does not handle all the constructs.
The strategy need to be changed to group the lines and organize
indentation by function. This latter is a better approach to handle
function expression, currently not yet implemented.
Simplify source code generator for expression lists
Accept non local function declarations in AST and bc code generator

Previously non local function declarations were stored in the AST tree
as a function expression plus an assignment.
Better code factorization in bytecode generator

Introduce a method "expr_tolhs" to generate and store an expression is a
given LHS location.
Generate code by prototype in code generator backend
Implement more statement rules for lua code generator backend
Improve rule for table literals in codegen
Add rule for "for .. in" construct in codegen
Complete the implementation of lua codegen backend

Pass all the tests except the "lexer-number-1" because of a complex
number representation problem.
Add a test to check expressions versus operator's prority
Add information about the Lua code generator backend
razionalize the syntax tree and its validation code

Remove the node of kind "BlockStatement". Now the blocks are just
list of statements.
Remove unused entry in AST/parser to build block statements
Merge remote-tracking branch 'lang-toolkit/master' into mat-safe-index

Changes includes:

- new lua code generator backend
- syntax tree simplification, eliminate BodyStatement node
- create file "syntax-dir.lua" to create AST nodes w/o validation
Implement CheckIndex rule in lua code generator
Implement rules in Lua code generator for Matrix and MatrixSlice

The rule for MatrixSlice is wrong when the slice appears in the left
hand term of an assignement. Cannot be fixed easily and need a change
in the approach of the lua code generator.
Fix syntax's rules for matrix constructs