toHash => pure, nothrow, const, @safe

[Walter Bright]

Consider the toHash() function for struct key types:

http://dlang.org/hash-map.html

And of course the others:

const hash_t toHash();
const bool opEquals(ref const KeyType s);
const int opCmp(ref const KeyType s);

They need to be, as well as const, pure nothrow @safe.

The problem is:
1. a lot of code must be retrofitted
2. it's just plain annoying to annotate them

It's the same problem as for Object.toHash(). That was addressed by making those 
attributes inheritable, but that won't work for struct ones.

So I propose instead a bit of a hack. toHash, opEquals, and opCmp as struct 
members be automatically annotated with pure, nothrow, and @safe (if not already 
marked as @trusted).