Notes on cmx loading and saving in Lambda_to_flambda

[lthls]

• Basic work: add a third kind of cmx files to cmx_format (in addition to the Closure and Flambda ones), save them in classic mode, and load them during Lambda_to_flambda in classic mode. This also means some saving and restoring of table entries, and dealing correctly with packs; the simplest way would make the Flambda_cmx_format.t type parametric, with one field containing either the approximations for Lambda_to_flambda or the full information from simplify. This introduces one extra indirection, but otherwise should allow us to reuse all of the tricky parts of Flambda_cmx_format. Overall, not a great deal of code to patch but without prior knowledge about which code needs to be used this could take several days at least.
• One way compatibility: this means a function creating typing environments from approximations. This should not be too hard, one or two days for someone with a good knowledge of the type-related code, perhaps up to a week for someone less familiar with that part of the code.
• Two way compatibility: generate approximations from typing environments. This could be done either on import, or systematically on export (to reduce the cost of loading in classic mode). This requires a deeper understanding of the types, but assuming that no unexpected problems show up it could be done in a week or so for someone already familiar with the code.