Add accessors to the VCDM `Context` type.

[timothee-haudebourg]

Improves the interaction with the VCDM Context type by adding methods to access, mutate and iterate over the context entries. Also add conversions from/to the json_ld::syntax::Context type.

• Add Context::contains method.
• Add Context::iter method.
• Add Context::insert method.
• Impl IntoIterator for &Context.
• Impl IntoIteratorfor Context.
• Impl Extend for Context.
• Impl From<Context> for json_ld::syntax::Context.
• Impl TryFrom<json_ld::syntax::Context> for Context.