Nested handlers

[guicho271828]

the first 3 commits are the same as #2 .
The main contributions are:

1. Added the nested bindings analogous to handler-bind.
2. Allowed multiple handlers for the same signal.
3. Multithreading support.
4. Toplevel handlers (similar to the previous implementation and *debugger-hook* in condition system, but always superseded by signal-handler-bind)

They required changes in the handler binding structure and how the bindings are applied.

For multithreading support, one-big-global-hash-table for all handlers are turned into dynamic thread-local bindings.
Also, when we implement nested handler bindings (based on nested special binding), making a hash-table for each nesting is space-inefficient. As a result, I used simple alist.

[fukamachi]

@guicho271828
Thank you for your contributions.

Actually, I don't use this library really often and can't be sure its quality high enough.
Could you maintain this library? I'll transfer this repository if you don't mind.

[guicho271828]

alright. I use this library often, so that might be reasonable.

[guicho271828]

BTW, do you have any idea implementing something like cl21.posix ?
good posix wrapper would be something everyone needs.

[fukamachi]

Okay, I'm going to transfer.

BTW, do you have any idea implementing something like cl21.posix ?

No ideas yet.

[fukamachi]

Ooops. It seems you need to delete your forked copy.

[guicho271828]

removed mine.
afaik trivial-signal is already in quicklisp, so we have to tell zach about the transfer.

[fukamachi]

I requested to transfer. Approve it.