Fix mac module load address

[Jake-Shadle]

• Fix module base address by always using a correct slide
• Make mac crash context optional
• Add some more API documentation

When calculating the image slide, the difference between the image's preferred load address and the TEXT segment, we faithfullly ported the Breakpad implementation:

breakpad

https://github.com/google/breakpad/blob/335e61656fa6034fabc3431a91e5800ba6fc3dc9/src/client/mac/handler/dynamic_images.cc#L269-L272

minidump-writer

minidump-writer/src/mac/streams/module_list.rs

Lines 97 to 101 in 1a1e912

	let slide = if seg.file_off == 0 && seg.file_size != 0 {
	image.load_address as isize - seg.vm_addr as isize
	} else {
	0
	};

This was, however, a mistake, as that logic was just incorrect as shown by the crashpad implementation

crashpad

https://github.com/chromium/crashpad/blob/df86075acc33314e611b351b33bf1c671b8cbc2f/snapshot/mac/mach_o_image_reader.cc#L585-L589

When removing the if and unconditionally calculating the slide, the images_match test that I added started passing which gives me a bit more confidence that the implementation is (more) correct now than the one copied from Breakpad.

While I was here, the test code needed a bit of refactoring in the minidumpwriter since that was easier than pulling out the logic into a reusable piece (it's maybe a good idea to do....but I also don't want us to end up in a crashpad scenario with a mountain of obtuse abstractions), so this addresses the Mac part of #37, and I'll do the same for Windows in a separate PR to close the issue.

Part of: #37
Resolves: #43