Remove periods in the canonical type name for Timestamps and Duration in Kotlin. (#1114)
Previously, optional timestamps would generate, eg:
`internal fun writeOptionaljava.time.Instant(v: java.time.Instant?, buf: RustBufferBuilder) ...`
and code which calls it:
`writeOptionaljava.time.Instant(v, buf)`
which upset the Kotlin compiler, causing errors like:
`Unresolved reference: writeOptionaljava`