compiler/studio: Add inline constants

From within an Aseba script, a new constant can be created using the
"const" keyword. Here are the rules:

- These constants are local to the node
- Constants must be declared first, prior to any variables
- Constants must have an initial value, resolving to a constant
  expression
- Obviously, the value cannot be changed afterwards
tests: Add unit tests for the new inline constants
Merge branch 'dev_inline_constant'