JIT: Return both size and pointer for finalized functions in JITModule

#### Feature

I want `get_finalized_function` in cranelift_jit's JITModule to return both the pointer and size to JIT'd functions, like `get_finalized_data` does for data object.

#### Benefit

This would allow me to disassemble individual compiled functions without having to guess at their size, making it easier for me to debug my compiler. Right now I dump them to standard output one byte at a time until my test program segfaults and manually locate the true end of the function code, which is unsustainable.

#### Implementation

`get_finalized_data` already works this way and seems to deal with the same type (`CompiledBlob`), so it should basically amount to copying that.

#### Alternatives

Adding a second function for getting the size would also work, but then the API would be inconsistent with itself.