Update slow buffer creation calls (#456)

[oleiman]

cherry-picked from bce35cd

Conflicts:

• ES6 updates (e.g. var -> let)
• platform lib
• fully excising 'buffer'

[oleiman]

Hi @mtth - per discussion on #449, figured I'd throw this over the wall since I'm using it on my fork. Apologies if I've sidestepped any process you have in place.

[mtth]

Thanks @oleiman, apologies for the very belated response. SlowBuffer constructor calls have been removed in v5.7.8 (via https://github.com/mtth/avsc/pull/499).