Add occursin and find* methods replacing search and findin

[nalimilan]

Add OccursIn predicate which replaces findin and all specific findfirst/findnext/findlast/findprev methods for specific types. These all call search or findin, but return nothing instead of 0 where appropriate. Note that there is an inconsistency with generic find* methods which already existed in Julia 0.6 and which continue to return 0: separate Compat.find* methods will have to be introduced for them to avoid conflicts with Base (#457).

Also fix in(::CartesianIndex, ::CartesianIndices) bug uncovered by importing in.

[nalimilan]

I'm not sure it's actually a good idea to return nothing rather than 0 from these functions. That means that changing the type of the element or of the collection would return a different value when there's no match, which makes it hard to write generic code. So I guess we should return 0 for all methods, and provide Compat.find* methods which return nothing.

The same discussion applies to findnext(c::EqualTo{Char}, s::AbstractString, i::Integer), but there its less clear since the method is less generic (less risk that types would change).

[stevengj]

I would suggest providing only the Compat.find* functions that return nothing. No need to provide new functions that return 0.

[nalimilan]

Well, several methods return ranges, so we can override the Base functions for them: it sounds too bad to force people to litter their code with Compat. when we could avoid it. Only methods returning scalars are problematic.

Also, I've realized that equalto and occursin (from Compat) work with the functions from Base, they are just going to be slow. So we may as well provide the fast methods since we provide these types.

I've pushed a commit adding unexported functions which return nothing. Methods are still added to Base functions, and they return zero. That sounds simpler both in terms of implementation and in terms of description of the behavior (see additions to README).