Add type annotations and optimize urwid.font

[penguinolog]

• optimize separate_glyphs: do not mutate gl, less temporary variables
• add __repr__ and human-friendly exception reraise on render
• add type annotations to satisfy mypy

Partial: #406

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

[penguinolog]

double transcoding is a bit overkill, just use strings directly

[penguinolog]

Font list output (changes: repr for symbols, so space characters will be not "invisible"):

Available Fonts:     (U) = UTF-8 required
----------------
Thin 3x3                  Characters: ' !"#$%\'*+,-./0123456789:;=?[\\]^_`'
Thin 4x3                  Characters: ' !"#$%\'*+,-./0123456789:;=?[\\]^_`'
Sextant 3x3          (U)  Characters: ' !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`'
Sextant 2x2          (U)  Characters: '%,.0123456789'
Half Block 5x4       (U)  Full ASCII + '´'
Half Block 6x5       (U)  Characters: ' ./0123456789:'
Half Block Heavy 6x5 (U)  Characters: ' ./0123456789:'
Thin 6x6                  Full ASCII
Half Block 7x7       (U)  Full ASCII + '´'