Remove trust_partner attribute from rpc connectors

This was only used for an optimization that is removed.

Remove ambiguous sized types from CakeML connector

Using fixed size types across different languages makes the marshalling
and unmarshalling code more predictable as it removes the need to call
out to a C compiler to calculate a platform specific width.

cakeml: use meta_utils lib to remove duplication

Goodbye `derive_eval_thm_bytes`

templates: CakeML derive_eval_thm in rpc template

Our rpc template requires the derive_eval_thm function which is no
longer availabe after updating our CakeML toolchain. The
derive_eval_thm function has been ported to the rpc-connector
template.

cakeml: add support for string inputs and outputs

Guard multi-language connectors

Prevents templates from doing any rendering if they would not be instantiated. Even if a
template is not instantiated it still must be processed in its deterministic order, so
this prevents issues where a template would not be able to be processed.

CakeML seL4RPCCall-to

Provides an implementation of the 'to' side of the seL4RPCCall connection in CakeML.
The rpc-connector is extended to support generating its code in CakeML, although
the CakeML generation does not support all the features that the C version does.