use inline comparison operators instead of strings.Compare

[tylerkovacs]

In the comments from strings.Compare:

// Compare is included only for symmetry with package bytes.
// It is usually clearer and always faster to use the built-in
// string comparison operators ==, <, >, and so on.

And another comment within the implementation:

// NOTE(rsc): This function does NOT call the runtime cmpstring function,
// because we do not want to provide any performance justification for
// using strings.Compare. Basically no one should use strings.Compare.

[tylerkovacs]

I ran a few tests and average sort time was about 8% faster for sorting 1,000,000 DocumentMatches. Results were quite variable though so I'm not sure the results are predictable. Still, the comments from the source were enough to convince me that this change seemed worthwhile.

[abhinavdangeti]

Thanks for the contribution @tylerkovacs .
This looks good to me.