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
