[RFC] Yet another buffer usage API revision.

[iwoplaza]

I propose we change the buffer usage extraction API from .as* or as*(...) to .as('*').

An example:

const fooBuffer = root
  .createBuffer(d.f32, 123)
  .$usage('uniform', 'storage');

const fooUniform = fooBuffer.as('uniform');
const fooMutable = fooBuffer.as('mutable');
const fooReadonly = fooBuffer.as('readonly');

Benefits:

• We can safely type the input to the as method, rather than having to change the output based on available flags.

[mhawryluk]

I like that this way it is more consistent with the .asView api for textures

[mhawryluk]

done in #836