Fix drop order of tls and statics and avoid double panics on deadlock

[Pointerbender]

Hello! I ran into some of the same issues as mentioned in #152, #179 and #291 and developed a fix for all of those issues. In a nutshell, the changes are:

• clarify the documentation of the mock lazy_static implementation regarding its Drop implementation
• ensure that global statics are dropped after thread locals
• ensure that loom does not panic inside a panic when dropping thread locals on the main thread
• add two regression tests for #152
• ensure that thread locals are destructed when a spawned thread panics
• ensure that loom doesn't double panic on deadlock
• add a test to verify that thread locals are properly destructed on spawned threads
• sprinkle comments generously everywhere to explain the code changes

This is my first time contributing to loom, please let me know if I can somehow improve this PR or if additional information is needed. Thank you!

[Pointerbender]

Hello! Just checking in to see if there is anything I can do to help shepherd this PR along. :) I'd be happy to provide extra details or hop on chat/zoom to review together if that saves some time. No rush, though!

[hawkw]

Thank you for working on this, this is great! I'll try to give it a review at some point soon, although I imagine @carllerche will want to as well.

[Pointerbender]

I've addressed the PR comments left by @adrianheine (thanks for the review!), are there any next steps left before this could potentially be merged? :) Please let me know if I can be of any further assistance. Thank you!