Fix corefile environment variables and speed up ELF.string()

[zachriggle]

I'm not sure how the logic for environment variables was supposed to work, but it was broken.

This fixes the environment, and ensures that embedded = bytes in the env value (e.g. FOO=BAR=BAZ) work correctly.

A performance enhancement for ELF.string(...) which reads a page at a time, rather than a byte at a time.

A change to corefile lifecycles such that if rename_corefiles=True, the original is deleted. This is to avoid the issue where core_pattern=core and there may be a race condition when two corefiles are created rapidly.

Some doctests were added to test these edge cases, including a corrupted stack.

[zachriggle]

Technically this is present in the 3.9 release as well, but the 3.10 release is coming up in a few days so we may as well just defer cutting a new release.

I'm still merging this to stable and merging through beta and dev.

[zachriggle]

These doctests definitely work locally for me, I'm not sure why they're failing.

[zachriggle]

So it looks like the issue is that we're still getting the old corefile.

**********************************************************************
File "elf/corefile.rst", line ?, in default
Failed example:
    core.pid, core.path, core.exe, io.executable
Expected nothing
Got:
    (6946, '/home/travis/build/Gallopsled/core.6946', Mapping('/tmp/pwn-asm-yA9YlO/step3', start=0x10000000, stop=0x10001000, size=0x1000, flags=0x7), '/tmp/pwn-asm-yA9YlO/step3')
**********************************************************************
File "elf/corefile.rst", line ?, in default
Failed example:
    core.pid, core.path, core.exe, io.executable
Expected nothing
Got:
    (6946, '/home/travis/build/Gallopsled/core.6946', Mapping('/tmp/pwn-asm-yA9YlO/step3', start=0x10000000, stop=0x10001000, size=0x1000, flags=0x7), '/tmp/pwn-asm-ERFKdo/step3')
**********************************************************************

Note that the exe path is different from what the corefile thinks it should be -- ERFKdo vs yA9YlO.

I've fixed this in c491be4 by unlinking the original when we do the rename.

[zachriggle]

Coverage and Travis pass. Codacy is disabled for stable (?), so I'm merging.