Implement assist to replace named generic with impl

This adds a new assist named "replace named generic with impl" to move
the generic param type from the generic param list into the function
signature.

```rust
fn new<T: ToString>(input: T) -> Self {}
```

becomes

```rust
fn new(input: impl ToString) -> Self {}
```

The first step is to determine if the assist can be applied, there has
to be a match between generic trait param & function paramter types.

* replace function parameter type(s) with impl
* add new `impl_trait_type` function to generate the new trait bounds with `impl` keyword  for use in the
  function signature

crates/syntax/src/ast/make.rs

@@ -232,6 +232,10 @@ pub fn impl_trait(
     ast_from_text(&format!("impl{ty_params_str} {trait_} for {ty}{ty_genargs_str} {{}}"))
 }
 
+pub fn impl_trait_type(bounds: ast::TypeBoundList) -> ast::ImplTraitType {
+    ast_from_text(&format!("fn f(x: impl {bounds}) {{}}"))
+}
+
 pub fn path_segment(name_ref: ast::NameRef) -> ast::PathSegment {
     ast_from_text(&format!("type __ = {name_ref};"))
 }