Previously, 'serde_json::Value' was used to store the serialized
template context. This value does not represent all of serde's data
model. This means we may fail to serialize a valid Rust value into it,
for instance, 128-bit integers. This is reduced with Figment's 'Value',
which supports the majority if not all of the serde data model.
At present, all supported templating engines use 'serde_json::Value', so
in practice, this commit has no effect but to reduce local dependencies
and provide better error messages for bad contexts.
This follows the completed graduation of stable contrib features into
core, removing 'rocket_contrib' in its entirety in favor of two new
crates. These crates are versioned independently of Rocket's core
libraries, allowing upgrades to dependencies without consideration for
versions in core libraries.
'rocket_dyn_templates' replaces the contrib 'templates' features. While
largely a 1-to-1 copy, it makes the following changes:
* the 'tera_templates' feature is now 'tera'
* the 'handlebars_templates' feature is now 'handlebars'
* fails to compile if neither 'tera' nor 'handlebars' is enabled
'rocket_sync_db_pools' replaces the contrib 'database' features. It
makes no changes to the replaced features except that the `database`
attribute is properly documented at the crate root.