Further optimize YANG statement parsing

Further optimize YANG statement parsing

Recursive definition of stringPart can easily end up with a large
number of retained objects -- i.e. we get StringPartContext for
each concatenation, with potentially small lists of three tokens.

Furthermore each unquotedString has ends up being typically defined
by a UnquotedStringContext containing a single StringPartContext,
which is clearly wasteful.

The grammar we are looking at is not that complex, hence we can
rewrite it without relying on recursion -- thus improving memory
footprint.

JIRA: YANGTOOLS-1089
Change-Id: I18eb1305d3e024d530552ac5cea0b5a928da0048
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>