Allow local modules and imports

[yannbolliger]

Closes #146.

All use statements are now allowed but a warning is emitted for "unknown" ones. E.g. the stainless crate, crate-local module imports and some things from std are ignored, all others get the following warning:

warning: Unknown use statement. Stainless likely cannot deal with the imported items.
 --> src/file.rs:1:1
  |
1 | use std::collection::HashMap;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

• Create test cases.
• Allow local use, mod. Warn for external use.