Add support for the client storage extension

[jrmuizel]

This change is 

[jrmuizel]

I've filed brendanzab/gl-rs#429 about adding a private field to the generated structs. We might as well try to do that first so we only need to bump the major version now and not next time.

[bors-servo]

☔ The latest upstream changes (presumably #128) made this pull request unmergeable. Please resolve the merge conflicts.

[kvark]

Thanks! Could you rebase and squash please?
I think we'll still need to wrap up those ffi calls (to client storage and such) in rusty functions, but that would be non-breaking follow-ups.

[jrmuizel]

I'd like some more progress on brendanzab/gl-rs#429 before I try rebasing this.

[jrmuizel]

I've dropped the version change commit now that gl_generator has been updated to include a private field.

[kvark]

@jrmuizel let's switch gl_generator dependency to 0.6.1 and bump the minor version (of gleam) here?

[jrmuizel]

Done

[kvark]

Thanks! Squash it, please?

[jrmuizel]

Why? Aren't they separate things?

[kvark]

It seems weird to have a multiple micro commits (one liners) following each other. Just because they can be treated independently doesn't mean they should? Anyhow, I don't have strong feelings about this.