The change updates the logic to determine if a function parameter is
valid for replacing the type param with the trait implementation.
First all usages are determined, to check if they are used outside the function
parameter list. If an outside reference is found, e.g. in body, return type or
where clause, the assist is skipped. All remaining usages only appear in the
function param list. For each usage the param type is checked to see if
it's valid.
**Please note** the logic currently follows a heuristic and may not cover
all existing parameter declarations.
* determine valid usage references by checking ancestors (on AST level)
* split test into separate ones
This removes an existing generic param from the `GenericParamList`. It
also considers to remove the extra colon & whitespace to the previous
sibling.
* change order to get all param types first and mark them as mutable
before the first edit happens
* add helper function to remove a generic parameter
* fix test output
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
