convert ExportLocation to ShortByteString to avoid PINNED memory fragmentation

This adds the overhead of a copy whenever converting to/from ExportLocation and
ImportLocation.

borg: Some improvements to memory use when importing a lot of archives.
(It's still pretty bad.)

Sponsored-by: Mark Reidenbach on Patreon

doc/bugs/borg_special_remote_memory_usage_high_for_large_borg_repo/comment_3_de81876865308a0c707011bcdb0ba843._comment

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+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 3"""
+ date="2021-10-05T17:39:29Z"
+ content="""
+A heap profile shows that the problem is an accumulation of PINNED memory.
+Both the memory used by borg list and by constructing the git tree.
+
+At least the borg list part seems very similar to the problem described here.
+<https://well-typed.com/blog/2020/08/memory-fragmentation/>
+The borg list gets read into a lazy bytestring, then it's split up
+and copied into strict bytestring chunks. But those get bundled
+back up into larger memory allocations as explained there. Then the files
+that are not git-annex objects are filtered out, resulting in memory
+fragmentation.
+
+I tried throwing in some S.copy in the borg list and filter part. Didn't
+help.
+
+I converted ImportLocation to use a ShortByteString, and that solved, 
+or at least improved, the borg list part of the problem. With 20 borg
+archives with 10000 annex objects each, the heap profile which had
+showed around 90 mb, mostly PINNED during that first stage, went down
+to 8 mb, none PINNED. (Although looking at the git-annex process 
+from outside, it still allocated 120 mb or so.)
+
+That leaves the memory use when constructing the git tree.
+Which would also probably affect importtree special remotes,
+when they have a large number of files.
+"""]]