runtime: two implementations of hasprefix

[bontibon]

runtime defines hasprefix(string, string) bool in string.go and hasPrefix(string, string) bool in type.go. They have identical implementations.

I do not know how they are used, but I imagine that either (a) one should be removed in favour of the other, or (b) document why both need to exist.

[gopherbot]

Change https://golang.org/cl/115835 mentions this issue: runtime: replace calls to hasprefix with hasPrefix