Binary: Make lazyGet more thread-safe

All values read lazily from the same BinHandle share the same mutable
offset variable (_off_r). If two such lazy values are simultaneously
evaluated, the two threads will step over each other when writing to
_off_r.

Fortunately, for BinMem handles, making lazyGet thread-safe is simple:
just use a fresh off_r variable when deferring the call to getAt.

For BinIO handles, a race condition still exists because IO handles
contain their own mutable file pointer variable that gets clobbered in a
similar way that _off_r would. But GHC doesn't use BinIO handles anywhere
so this particular issue could be ignored for now.

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