Let dim ids share the same domain with var ids and formal parameters

[lucafavatella]

Main changes:

• Let dim ids, var ids and formal parameters share the same domain;
• Implement lexical scoping between dim ids, var ids and formal parameters;
• Improve and refactor test suites.

This pull request depends on pull request #45.

TODO items introduced by this pull request:

• The following expression evaluates to 1 as upstream TL, but needs to be carefully considered for correctness of optimized C cache indexed by numbers:

#.d @ [d <- 1] where var d = 0 end

• The following expression returns the hidden unique dimension: shall the hidden dimension be exposed?

F.1 where fun F.x = x where dim x <- 46 end end

• The following two expressions both return 'badarith' error, some thought/work is needed in order to let missing dimensions be more clearly identified:

(F.46 where fun F.x = x + #.x end) where dim x <- 1 end
F.1 where fun F.x = (x + #.x) where dim x <- 46 end end