Rework build into a class

Rework build into a class

This simplifies usage, goes from

    build['flag']
to
    build.flag

Also makes static analyzers able to detect invalid attributes as the set
is now limited in the class definition.

As a bonus, setting of the default field values is now done in the
constructor, not separately and manually.

While at it, unify "build", "thisbuild", "info", "thisinfo", etc into
just "build".