Deprecate getindex/checkbounds methods for non Integer Real indices for Chars and Strings

[ScottPJones]

These functions depended on a version of to_index, which has been deprecated.
I tried to add tests for these methods, because they showed up as not being covered,
however I was told not to, because they give a deprecation warning.
This now gives a better error to the user, giving a work-around, and also giving the method that they called that doesn't work any longer, and eliminates the coverage holes in strings/basic.jl and char.jl

[JeffBezanson]

+1

[simonster]

I may be missing something, but is there a reason these need to call depwarn explicitly instead of being defined using the @deprecate macro?

[ScottPJones]

Probably not, I suppose @deprecate could be used, I was thinking about having the warning different than the default originally.

[ScottPJones]

Bump: this now uses @deprecate instead of depwarn, passes tests, should be mergeable. Thanks!

[jakebolewski]

LGTM

[ScottPJones]

Big thanks to @jakebolewski for shepherding my coverage tests in.