This commit introduces a more intuitive syntax for ranges,
allowing the user to write `[0 .. 4]` or expressions such as:

- `(l[0] + .. + l[n - 1]) / len(l)` to compute the mean of `l`, or

- `n * (n - 1) * .. * 1` (to compute `n!`).

See README.md for more details on the allowed syntax.

The commit contains a large part of refactoring to split values
representation between `FrontValue` for values constructed by the
parser, and `Value` for "desugared" values, transformed by a
preprocessing phase in `ast.rs`. This refactoring opens the way for
introducing other syntactic sugars.