WIP: get rid of module rec and functors by moving many types into types.ml

[lefessan]

Currently, the code of alt-ergo is hard to enter because it is a huge recursive mess, where all theories are functors with a Sig.SHOSTAK interface and they are all gathered together in the shostak.ml using a big module rec.

This patch is probably not intented to be merged at any point, but it might be interesting as it simplifies the code by removing this use of functors and module rec :

• Types from symbols.ml, expr.ml and all theories are gathered together in a file types.ml (which makes it easier to understand what they are ...)
• A shostak_pre.ml file exposes shostak.ml functions before they are available, to allow a huge recursion between modules. It exposes a set of refs, which are set from the shostak.ml file at the end.
• Most of the changes are replacements of scopes changes to use Types instead of Symbols or Expr.