Add Decimal64.scaleTo()

Add Decimal64.scaleTo()

Decimal64.valueOf(String) results in variable-scale results, based on
the input string. This is sub-optimal when we want to compare values
of particular type -- those should be governed by fraction-digits and
hence, for example "2.00" and "2.0" should be normalized to the same
scale.

Introduce Decimal64.scaleTo(), which can be used to adjust the scale
of an existing Decimal64.

JIRA: YANGTOOLS-1440
Change-Id: Icbc215dff6d8146996c5be1040751fd1e14b6cfe
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>