Emit generics as native templates in C++

src/bindgen/bindings.rs

src/bindgen/cdecl.rs

src/bindgen/ir/generics.rs

src/bindgen/ir/mod.rs

src/bindgen/ir/opaque.rs

src/bindgen/ir/specialization.rs

src/bindgen/ir/structure.rs

src/bindgen/ir/ty.rs

src/bindgen/ir/union.rs

src/bindgen/library.rs

src/bindgen/parser.rs

src/bindgen/writer.rs

tests/expectations/alias.cpp

tests/expectations/annotation.cpp

tests/expectations/cdecl.cpp

tests/expectations/cfg-2.cpp

tests/expectations/cfg.cpp

tests/expectations/constant.cpp

tests/expectations/enum.cpp

tests/expectations/euclid.cpp