fix: don't allocate for static strings in ereport!

[gruuya]

This is a spin-off from #2269; whereas in that PR we resolve the issue of eager allocation in high level logging macros (debug1, log, warning, etc.), there was one omission concerning only ereport!.

Namely, ereport! now uses IntoMessage::into_message to get the message from the input arg. When invoked indirectly via high level logging macros this is always std::fmt::Arguments, which in case of static strings results in a std::borrow::Cow::Borrowed message.

However, when ereport! is called directly with a static string this applies instead

pgrx/pgrx-pg-sys/src/submodules/elog.rs

Lines 131 to 136 in b2be3e1

	impl IntoMessage for &str {
	#[inline]
	fn into_message(self) -> std::borrow::Cow<'static, str> {
	std::borrow::Cow::Owned(self.to_owned())
	}
	}

In other words, in this case we always allocate, even for static strings. The fix is simple: replace impl IntoMessage for &str with impl IntoMessage for &'static str (since Rust doesn't allow both to co-exist), which can then return std::borrow::Cow::Borrowed for ereport!(..., "my string literal").

The only "downside" is that stuff like

      let msg = format!("table {} not found", table_name);
      ereport!(..., msg.as_str());

will error out during compilation. Arguably though this is for the better, since it forces the user to pass the string down, or call clone()/to_string()/to_owned(), making the allocation explicit.