Allow ThreadLocalVar to be initialized with a block

[ivoanjo]

ThreadLocalVars can now receive a block in the #initialize method, which will be called to initialize the value on each thread.

This allows default initialization to work with non-value types, where
each thread gets its own copy:

v = ThreadLocalVar.new { {} }
h1 = Thread.new { v.value }.value
h2 = Thread.new { v.value }.value

h1.equal? h2 # -> false

[jdantonio]

Thank you so much @ivoanjo! This is fantastic work. I want to pull the code locally and experiment with it before I merge it in, but I expect I will merge it without changes.

[ivoanjo]

Thanks! I was inspired a bit by Hash which has a similar API (although without saving the default value after getting it from the block), and it really feels much more natural and high-level to use.

[ivoanjo]

Awesome!