Allow project_replace argument to be used without Replace

[taiki-e]

Currently, both Replace and project_replace = <name> arguments are required to name the return value of project_replace() method.

#[pin_project(Replace, project_replace = EnumProjOwn)]
enum Enum<T> {
    Variant(#[pin] T)
}

It is not ideal that we need to have both Replace and project_replace = <name> as project_replace = <name> implies that a project_replace method is generated.

This PR makes project_replace argument to use without Replace argument.

- #[pin_project(Replace, project_replace = EnumProjOwn)]
+ #[pin_project(project_replace = EnumProjOwn)]
  enum Enum<T> {
      Variant(#[pin] T)
  }

Also, makes project_replace argument an alias for Replace so that it can be used without a value.

#[pin_project(project_replace)]
enum Enum<T> {
    Variant(#[pin] T)
}

Related: rust-lang/futures-rs#2176 (comment) hyperium/hyper#2220 (comment)

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci

[taiki-e]

Also, the Replace argument will be deprecated in the future. (There is no way to generate a warning for an attribute argument, so just remove it in the next major version.)