Rename "count" to "countPrefix"

[chexxor]

With the next release of breaking changes to this library, the "count" function should be given a better name. My expectation of "count" function in context of a string is to tell me how many characters are in that string, but this "count" function actually counts the prefix characters which satisfy a predicate.

One suggested name is "countPrefix".

Issue originally raised in #79 (comment)

[garyb]

Resolved in the 0.12 branch