Optimization: RAW display block read

[penguinolog]

Instead of reading per-byte, we can read per-chunk and make fewer read cycles. With chunk size 1024 bytes, we have almost 100% warranty of single-cycle buffer read

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Pull Request Test Coverage Report for Build 7738568816

• -22 of 27 (18.52%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 70.968%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
urwid/display/_raw_display_base.py	1	3	33.33%
urwid/display/_posix_raw_display.py	2	12	16.67%
urwid/display/_win32_raw_display.py	2	12	16.67%

Totals
Change from base Build 7725240357:	-0.1%
Covered Lines:	8769
Relevant Lines:	12447

---

💛 - Coveralls