Special-case identifier lexer token

Special-case identifier lexer token

An unquoted string can be an IDENTIFIER, which is the most common
case anyway. This token does not have to be further validated and
thus it makes sense to special-case it. This has the neat effect
of reducing the depth of our parse tree, too -- bringing memory
usage down by 6.6% versus the original fix.

JIRA: YANGTOOLS-1089
Change-Id: I63701719e90334558048201d70d8264ab9299efb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 0ec7fd50a8d7768f55d03e2ad1c6156fb6836c67)