Implement unic-ucd-block

[behnam]

Provides access to char property:

• Block (Enumerated)

Also let's have two iterators:

• All assigned/defined Unicode Blocks
• A character iterator that only walks on character ranges inside these blocks. This is very helpful for reducing the range of chars we test in integration tests, which is wasting a lot of CPU also walking on the ~75% of the code space that's not assigned.

References:

• http://www.unicode.org/reports/tr44/tr44-4.html#Script
• http://www.unicode.org/Public/UCD/latest/ucd/Blocks.txt

Consistency tests:

• Any character assigned should have a block.
• Some blocks contain unassigned characters.

Cross-component tests:

• IIRC, each Block can contain characters from one non-Common/Unknown Script. (GH-137)

Similar crates: N/A

[eyeplum]

While #217 is waiting for code review, I will try to implement this one.

[behnam]

Thanks, @eyeplum! I already have a half-baked implementation on my laptop. If you haven't started yet, maybe wait a few days before you start?

[eyeplum]

@behnam Sure!

[behnam]

Thanks, @eyeplum, for wrapping this up! 😄