Fix String value parsing/serialization

When encoding user-supplied values from user we need to be mindful of
their actual content: we cannot just use single quotes (') everywhere,
as the value itself can contain a single quote, in which case the
resulting string would have unbalanced quotes and nominally unparseable.

Adjust both sides of the serialization picture to account for this and
use YANG section 6.1.3 for escaping rules.

JIRA: YANGTOOLS-1473
Change-Id: I0df9beaf720a78682b6bde606a1359e7f1a09df8
Signed-off-by: Ruslan Kashapov <ruslan.kashapov@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>