Fix bugs, compiler warnings and add tests

[birktj]

This PR adds a Limits struct to fix the memory exhaustion bug in #29, this changes the public API slightly, because ifd::Entry::val now also needs a Limits parameter. It adds some array bounds checking for indices that directly come from the tiff file. While I was at it I also fixed #13 by changing Decoder::read_string to return a TiffResult instead of a FromUtf8Error result. I also fixed some smaller compiler and clippy warnings.

I added the samples from #28, #29 and #31 as new tests, thank you @Shnatsel!

This will fix #13, fix #28, fix #29, fix #30 and fix #31.

@Robzz and @HeroicKatora what do you think of this?

[Shnatsel]

Thanks for fixing these so quickly! This has unblocked further fuzzing which found at least one more panic: #33

[birktj]

@HeroicKatora I have fixed the possibly overflowing multiplication and changed the limits to usize. Do you think I need to change anything else or are we good to go?

[birktj]

@HeroicKatora @Robzz @bvssvni friendly ping. Would it be possible to look at this so that we can eventually release a new version of the library with the newly merged PRs?

[HeroicKatora]

Just now realized that during converting to TiffResult, the line you changed here https://github.com/PistonDevelopers/image-tiff/pull/32/files#diff-ee033e65f37f7717cb17dc37bc71de6bL301 was actually a bug as the error was ignored. Good job 👍 We should probably do a stability release sometime soon. Want to merge this today if no further concerns appear.