This commit stops converting non-string inputs to strings when
parsing ISO strings or property bag fields.

When numbers are coerced to strings, the result is sometimes a valid
ISO string subset but it often doesn't behave as expected. The result is
often brittle, yielding "data driven exceptions" that we've tried to
avoid.

Numbers can also be ambiguous, e.g. is a Number passed for
`offset` mean hours like an ISO string, minutes like
Date.p.getTimezoneOffset, or msecs like Date.p.getTime().

This commit also removes coercing objects to strings in the
same contexts (like TimeZone and Calendar constructors) because
it's unclear that there are use cases for this coercion.