New LD context import mechanism.

[timothee-haudebourg]

This PR introduces a new method for importing external JSON-LD contexts into the generator. Importing external base context is required to avoid generating definitions for terms that are already defined or protected in the base context.

Before this PR, three differents representations for LD contexts were coexisting insinde the generator:

• one for local contexts generated from TreeLDR layouts,
• one for processed exteral LD contexts,
• a last one for unprocessed external LD contexts (local contexts).

This caused a number of complications in the codes, in particular with unprocessed local contexts. This lead to various bugs triggered by incorrect or inconsistent interactions between the contexts.

With the new import mechanism, all contexts share the same internal representation. A first static analysis is performed to resolve dynamic values introduced by unprocessed contexts. Even if it complicates the first stages of the generator, this greatly simplifies the generation itself by allowing a unique and consistent comparison of contexts and term definitions.

All the tests are passing, and this implementation can still generate a context for Rebase, Verite. In addition it can generate a context for Open Badges.

What is missing

• Error messages (for now it panics)
• Context loading. For now imported contexts must be inlined and not referenced. It is just a matter of integrating with json_ld::Loader.