gccrs: derive(PartialEq): Also derive StructuralPartialEq

gccrs: derive(PartialEq): Also derive StructuralPartialEq

gcc/rust/ChangeLog:

	* expand/rust-derive-partial-eq.cc: Adapt signatures to generate two impls.
	* expand/rust-derive-partial-eq.h: Likewise.
	* expand/rust-derive.cc (DeriveVisitor::derive): Adapt to multiple item generation.

gcc/testsuite/ChangeLog:

	* rust/compile/derive-eq-invalid.rs: Declare StructuralPartialEq.
	* rust/compile/derive-partialeq1.rs: Likewise.
	* rust/execute/torture/derive-partialeq1.rs: Likewise.