Introduce address based index to ELF symbol table cache

So far our symbol table cache had the symbols mapped into memory and
then sorted references to them by the respective symbol's address, in a
heap allocated array. This is conceptually fine, but it is an approach
that is incompatible with future changes.
This change reworks the SymbolTableCache type as follows:
1) we to store the memory mapped symbols as-is; that is, we keep an
   (unsorted) slice of the symbols around
2) we then build an additional index that is sorted by address on top of
   that

In so doing we eliminate the need for keeping unnecessary references to
the memory mapped data around. Further more, we could optimize the (heap
allocated) index further, by using a smaller index size when the number
of symbols is below certain maxima. Third, because symbol filtering now
happens inside SymbolTableCache, we remove the duplication for filtering
and sorting symbols for 32 and 64 bit, respectively. Lastly, because we
no longer heap allocate an array of references, we can ditch the
ElfN_BoxedSyms type, which was arguably a weird special case to have,
entirely.

Signed-off-by: Daniel Müller <[email protected]>