Use ‘with-current-buffer’ rather than save-excursion+set-buffer

[diamond-lizard]

When byte-compiling version 0.10 of pg.el, I get the following warning:

In pg:lo-import:
pg.el:838:69:Warning: Use ‘with-current-buffer’ rather than
    save-excursion+set-buffer

In pg:lo-export:
pg.el:848:40:Warning: Use ‘with-current-buffer’ rather than
    save-excursion+set-buffer

In pg:read-char:
pg.el:942:35:Warning: Use ‘with-current-buffer’ rather than
    save-excursion+set-buffer

[diamond-lizard]

Fixed with commits:

• 36869c6
• 70acb36
• 5b7ef92
• 2f7af6d