* fix: untyped constant cconverson to default type
In definition assign expression, the source type is propagated to
the assigned value. If the source is an untyped constant, the
destination type must be set to the default type of the constant
definition. A fixType function is provided to perform this.
In addition, the type conversion and check of constants is
refactored for simplifications.
Fixes#727.
* test: fix _test/const14.go