[unsafe-fields] Support tuple structs

[joshlf]

While generating field names in macros would be tricky (since we'd need to synthesize 0, 1, etc), we don't actually need to use field names - we just need to use unique strings. We can do this by having the string input to the hash function be all types up through and including the given field. E.g., for the type:

struct Foo(Bar, Baz, Blah);

We would desugar into:

struct Foo(
    Unsafe<Foo, Bar, {hash_field_name("Bar")}>,
    Unsafe<Foo, Bar, {hash_field_name("BarBaz")}>,
    Unsafe<Foo, Bar, {hash_field_name("BarBazBlah")}>,
);