…r=Veykril

internal: make function builder create ast directly

I am working on rust-lang#17050.
In the process, I noticed a place in the code that could be refactored.
Currently, the `function builder` creates the `ast` through the `function template` , but those two processes can be combined into one function.
I thought I should work on this first and created a PR.

…r=Veykril

internal: make function builder create ast directly

I am working on rust-lang#17050.
In the process, I noticed a place in the code that could be refactored.
Currently, the `function builder` creates the `ast` through the `function template` , but those two processes can be combined into one function.
I thought I should work on this first and created a PR.

…-new, r=Veykril

feature: Make generate function assist generate a function as a constructor if the generated function has the name "new" and is an asscociated function.

close rust-lang#17050
This PR makes `generate function assist` generate a function as a constructor if the generated function has the name "new" and is an asscociated function.
If the asscociate type is a record struct, it generates the constructor like this.
```rust
impl Foo {
    fn new() -> Self {
        Self { field_1: todo!(), field_2: todo!() }
    }
}
```
If the asscociate type is a tuple struct, it generates the constructor like this.
```rust
impl Foo {
    fn new() -> Self {
        Self(todo!(), todo!())
    }
}
```
If the asscociate type is a unit struct, it generates the constructor like this.
```rust
impl Foo {
    fn new() -> Self {
        Self
    }
}
```
If the asscociate type is another adt, it generates the constructor like this.
```rust
impl Foo {
    fn new() -> Self {
        todo!()
    }
}
```