Centralise common backend routines and continue backend interface

[schnellerhase]

Centralises common backend logic for reusability in different backends.

• template_keys: extracts template keys, used for checking all keys matched by input data
• integral_data: computes unrolled sorted integral data of a FormIR
• tensor_sizes: computes the sizes of the tensors associated with the kernel invocation of a form or an expression

Consistently names the implementation files in singular (expressions.py -> expression.py, etc.)

All includes in code generation now pass though the module level, i.e. __init__.py of C/ or numba/ - no longer enforcing a given file structure to be present in the backend (that's an implementation detail). This continues the work to define the language backend interface.

"""Generation of MyBackend code."""

class FormatterImpl:
    def __init__(self, dtype: npt.DTypeLike) -> None: ...
    def __call__(self, obj: L.LNode) -> str: ...

class expression: # (or namespace)
    def generator(ir: ExpressionIR, options: dict[str, int | float | npt.DTypeLike]) -> tuple[str, str]: ...

class integral: # (or namespace)
    def generator(ir: IntegralIR, domain: basix.CellType, options: dict[str, int | float | npt.DTypeLike]) -> tuple[str, str]: ...

class form: # (or namespace)
     def generator(ir: FormIR, options: dict[str, int | float | npt.DTypeLike]) -> tuple[str, str]: ...

class file: # (or namespace)
    def generator(options: dict[str, int | float | npt.DTypeLike]) -> tuple[tuple[str, str], tuple[str, str]]: ...

The only thing that needs to be addressed/improved on after this for #805 is the handling of output files, the split into declaration and implementation (or header and source) should be generalised away from the backend.

[coveralls]

coverage: 84.503% (+0.003%) from 84.5%
when pulling b8e7042 on schnellerhase/backend-common
into dde9f73 on main.

[michalhabera]

Moving the language backend independent code looks good, but I am not in favor of flattening the module structure to generator_form, etc. If this is the implementation detail, why not force user to prepare the module structure with classes then?

[schnellerhase]

Reverted the flattening of the interface to free functions. Maintains the namespaced {file,expression,integral,form}.generator signatures.