Refactor YANG statement parser structure

Current definition leads to leading whitespace being considered
a part of the statement. This leads to misleading line/character
being reported as the start of the statement.

Fix this by introducing a top-level 'file', which encapsulates
at least one statement with its leading/trailing whitespace. The
other parts then come together naturally.

JIRA: YANGTOOLS-1129
Change-Id: I9f725b09eae6f8acf4397c97c60f99caaade798b
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit b462288cc6e678b9350551c64ecda96196ebe8b4)