no_std for symphonia-core: switch to core and alloc

[atoktoto]

Hello again,

This is the first, least intrusive step in the direction of supporting no_std build environment. I switched all std:: imports to their core:: or alloc:: counterparts leaving the rest untouched. This change should be 100% backwards compatible and make the further changes less noisy.

I intend to follow the same path as described in #244 but in smaller steps this time around :)

[sscobici]

use dev-0.6 branch instead of master as it has many changes.
I'm still not sure if no_std is in plan for 0.6.

[atoktoto]

ah, right. In that case I will close this one and rebase first.