[Pitch] String Input-validating Initializers

[benrimmington]

From your other recent pitch:

@glessard

A common case is a simple re-implementation of C's strlen()

If strlen is commonly reimplemented, then should it be in the standard library, possibly as an UnsafeBufferPointer initializer?

(It probably wouldn't validate as UTF-8/16/32, so maybe off-topic here.)