type 'a key should be string instead of Bytes.t

[vbmithr]

Internally, the type 'a key in several modules should probably by string instead of Bytes.t, because keys are supposed to be immutable and only exported/imported from storage (if I understand well).

[whitequark]

That would require either unsafe operations everywhere or copying, neither of which really improves anything.

[vbmithr]

Superseeded by #40.