This PR is based on the previous PR (#33), and tries to add support for the uniqueness extension.

• Add two new axes to the mode system. The uniqueness axis contains unique < shared. The linearity axis contains many < once. To accomodate the new axes, some refactoring and renaming are done. More of that is due but I think it's better to merge this ASAP instead of rebasing weekily.
• To check that variable uses declared to be unique are actually so, we add uniqueness_analysis.ml that checks the usage of each values (In particular, Texp_ident and Texp_field).

The extension still has many issues, our minimal goal in this PR is that existing code will translate to the same machine code when the unique extension is disabled. Existing code might be rejected by type checker due to some bugs we introduced, but that's less scary. To achieve this minimal goal, the following is needed:

• We first make sure that this PR doesn't change the locality axis in the alloc_mode in the typedtree generated by typecore.ml.
• In translcore.ml, add assertions that the two new lambda primitives (that overwrite fields in blocks), Preuseblock and Preusefloatblock are generated only when unique extension enabled.
• Note that in lambda.ml, we add uniqueness to alloc_mode. uniqueness could be either alloc_unique or alloc_shared. The former is redirected to the latter when the extension is disabled. Further inspection is needed to ensure that translcore.ml retains old behaviour when uniqueness is alloc_shared.