Allow context to be set via call

[willcosgrove]

This PR is doing more than one thing 🙈

If you want me to split this up, let me know. But in addition to allowing context to be passed via call, I've also moved where view_context is stored. It's now inside of the Phlex::Context instead of in an ivar of SGML.

I think that ultimately, view_context as a concept can be fully removed from Phlex, and only defined in phlex-rails which is the only place it gets used. But as I went down that path the changes got more and more complicated. phlex-rails then needs to redefine call, which means it needs to be tightly coupled to phlex's definition of call... it got messy.

Let me know what you think, and if you want me to leave @_view_context alone, that's fine too. I'll open a sibling PR over on phlex-rails to coincide with this change.