Simply move the test for `keywords::StaticLifetime` into the
`Lifetime` impl, to match how elision is checked.

Previously a `'static` lifetime bound would result in an `undeclared
lifetime` error when compiling, even though it could be considered
valid.

However, it is unnecessary to use it as a lifetime bound so we present
the user with a warning instead and suggest using the `'static` lifetime
directly, in place of the lifetime parameter.