Merge Overview and Motivation

[lukescott]

Made some comments on 83291b0. Some redundancy between the two.

I'll probably get to it at some point.

[Naddiseo]

As I noted on the commit:
I think they should be separate since they address different issues:
Overview -> What we're trying to accomplish.
Motivation -> Why we're proposing this, and what is the current problem.

I do agree, though, that the duplicate wording in the motivation section needs to change.

[Naddiseo]

After reading the motivation section again, let's just get rid of that last sentence; I think all of it is covered in the overview.

[Naddiseo]

The order of what is mentioned in the motivation should be reversed; I think the "string" + undefined behaviour isn't the primary reason/problem for this proposal.
In order of priority I think what we're trying to address:

• There are several slightly different / non-standardized ways of type hinting
• They lead to some ambiguities in syntax
• There are some strange behaviours with undefined which type hints + defaults can solve in come cases.