Add Brioche.get global in std

[kylewlacy]

Companion PR: brioche-dev/brioche#34

This PR updates the std package to define the new global constant Brioche, plus the function Brioche.get(...). This is a special function that works with the Brioche runtime to allow loading an artifact from a local path relative to the current module. It's defined as a global because the runtime needs special awareness of this function.

I tried to make the implementation pretty defensive when setting Brioche.get-- it's only set if it's currently undefined. I figured this might save some headache in the future if we end up in a state where we have multiple versions of std loaded at once (which will probably cause mayhem with this feature). If we want to handle that case, future versions of std can set Brioche.get globally, which will take precedence over this implementation.