Read/write files with proper coding conversion

Change f-read to decode the text according to Emacs' automatic coding
conversion.  f-read/f-write are now for text IO with implicit automatic
coding conversion according to Emacs' rules.

Add f-read-bytes/f-write-bytes for raw byte IO, and
f-read-text/f-write-text for text IO with explicit coding system.
Merge branch 'coding-conversion' into v0.6-wip