Improve specification conformance of unicode bidi library

Background information: Web browsers are expected to support international text, and Servo is no exception. We are writing a library to implement the Unicode Bidirectional Algorithm for display of mixed right-to-left and left-to-right text, and it has not yet achieved full conformance with the specification.

Initial step:

• clone the unicode-bidi repository, compile it, and run the tests in defined in src/lib.rs
• email the mozilla.dev.servo mailing list introducing your group and your progress
• add code to tools/generate.py to download the files that make up the conformance testsuite: http://www.unicode.org/Public/UNIDATA/BidiTest.txt and http://www.unicode.org/Public/UNIDATA/BidiCharacterTest.txt
• by hand, convert several test cases from the file into Rust tests that can be run automatically

Subsequent steps:

• add code to tools/generate.py that automatically converts the tests in the conformance suite into Rust tests that can be run automatically (https://github.com/servo/unicode-bidi/issues/1)
• implement the missing step L1 from the UBA (https://github.com/servo/unicode-bidi/issues/2)
• implement the missing step N0 from the UBA (https://github.com/servo/unicode-bidi/issues/3)
• solve the conformance problems related to the implementation of steps W1 to W7 (https://github.com/servo/unicode-bidi/issues/8)