Support for Yul functions in assembly blocks

For example

```
function allocate_memory() -> word {
let res : word;
assembly {
function allocate_unbounded() -> memPtr {
memPtr := mload(64)
}
res := allocate_unbounded()
}
return res;
}
```

No source file, test, or entry point is named. Start by locating the compiler's handling of assembly blocks and the existing parsing or type-checking tests, then use the provided Yul function example as the acceptance case: compilation should support local function declarations and calls inside an assembly block.

Written by the indexing model from the issue text.