compiler: make struct types more unique

There are a lot more fields that are important when comparing structs
with each other. Take them into account when building the unique ID per
struct type.

Example code that differs between the compilers:
https://play.golang.org/p/nDX4tSHOf_T

testdata/interface.txt

@@ -7,6 +7,8 @@ is Foo: 18
 is Thing: foo
 is *Thing: foo
 is *Thing: foo
+is struct{n int}
+is struct{n int `foo:"bar"`}
 is Doubler: 6
 is Tuple: 1 7 11 13
 Array len: 4