Define IfFeature via a specialized class

Define IfFeature via a specialized class

This reworks the grammar to use less recursion, as we can use
wildcards to eagerly combine expressions like "foo || bar || baz"
into any(foo, bar, baz) rather than or(foo, or(bar, baz)).

In order to properly support this contract, we define IfFeatureExpr,
which we specialize, so that we do not need to lug an opaque
predicate tree.

JIRA: YANGTOOLS-964
Change-Id: I048c3ced9c074e340031ac53c1117ce881b0a78b
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>