[host] Remove calls to fbl::AutoCall

fbl::AutoCall is deprecated in host/userspace in favor of fit::defer.

Calls to fbl::AutoCall could not be removed from kernel or system code
since they may not have the C++ standard library included.

TEST=runtests
CP-146: #progress

Change-Id: Ief2f908430c17851f9f40e4bbabbe122005033d6

[blobfs] Avoid double-counting duplicated blobs

During the image generation process, space is pre-calculated for the
underlying image based on the manifest of blobs which have been
provided. Since blobs are content-addressable and naturally duplicated
by blobfs, it is possible that multiple blobs with the same contents
will consume less space than they would individually.

This patch modifies the host-side tools to pre-generate the merkle
trees (once), find the unique set of blobs within them, and only
allocate / attempt to add blobs after they have been deduplicated.

This shrinks the generated "fvm.blk" image from 736 MB -> 394 MB
for the Astro build.

Test: host FVM tests, manually building
ZX-2141 #comment In Progress
US-441 #comment In Progress

Change-Id: I1ccdab91e74691e607f27419582dadbe7ced97e9

[blobfs] [minfs] [host] Combine common functionality between tools

De-dupe code between the minfs and blobfs host side tools as much as
possible. Additionally, add minfs support for adding manifests on create.

ZX-2033 #done

Change-Id: I0ece73d37927e6589ccd7d1ce235fc484668a221