Refactor simd API and add SVE backend (#1044)

* Add new API for the SIMD library that is compatible with the ARM 'sizeless' SVE vectors. Language restrictions prevent the use of the operator overload interface used up to this point for SIMD.
* Add `indirect_expressions` and `indirect_indexed_expressions` for describing memory reads/writes. `where_expressions` control masked access to simd vectors.
* Implement the SVE SIMD back-end in accordance with the alternate SIMD API.
* Retrieve vector width information from compiled mechanisms.
* Use alternate SIMD API in modcc-generated mechanisms.
* Add assertion in generated mechanism code that checks runtime vector width compatibility.

Fixess #1021.