Instead, we now have a looser helper called `checkContains(...)`
that will match on any occurrence similarly to `std.mem.indexOf()`.
While at it, I have cleaned up other combinators to make the entire
API more consistent, and so:
* `checkStart(phrase)` is now `checkStart()` followed by
`checkExact(phrase)`
* `checkNext(phrase)` if matching exactly is now `checkExact(phrase)`
* `checkNext(phrase)` if matching loosely is now `checkContains(phrase)`
* `checkNext(phrase)` if matching exactly with var extractors is now
`checkExtract(phrase)`
Finally, `ElfDumper` is now dumping contents of `.symtab` and `.dynsym`
symbol tables. I have also removed dumping of symtabs as optional - they
are now always dumped which cleaned up the implementation even more.
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