Common name for all GlobalCtx producing methods

[jimblandy]

I think Module::to_ctx and front::spv::BlockContext::gctx should both be named as_global_ctx:

• We should use a single name for this operation across the code base.
• Borrowing methods are generally called as_foo in Rust.
• ctx is too unspecific.

This was originally a comment I had on gfx-rs/naga#2335, but I think making the changes directly in those PRs will make rebasing a nightmare, so let's handle it as a followup.