fallback: Use mmap objects instead of strings to unpack

Signed-off-by: Bas Westerbaan <bas@westerbaan.name>
Revert "fallback: Use mmap objects instead of strings to unpack"

See next commit.

This reverts commit 770fed6b7f1ce1f6e9c20881897a417680c8b79a.
fallback: _fb_read: add fast-path