Create function fo recursive type convertion

[totikom]

Today's implementation is able to handle only bare types (such as MyType) or references to bare types (&MyType and &mut MyType). I want to implement a function, which will recursively go through syn::Type instance and convert all references to Boxes.
In addition, function for recursively borrowing struct fields must be implemented.

Example:

// For function:
fn foo(a:u32, b: &MyType, (c,d):(&AnotherType, &mut i64)) {}

// This struct and function will be generated
struct __fuzz_struct_foo {
a: u32,
b: Box<MyType>,
c: Box<AnotherType>,
d: Box<i64>,
}

pub fn __fuzz_foo(mut input: __fuzz_struct_foo) {
    foo(input.a, & *input.b, (& *input.c, &mut *input.d);
}