SQLiteDateParser: use withUnsafeMutablePointer instead of allocations
SQLiteDateParser: use withUnsafeMutablePointer instead of allocations

OK, not really: the nanosecond components remains in the heap.
But we may find a way to put 11 chars on the stack.

groue/GRDB.swift,groue/GRDB.swift,groue/GRDB.swift,groue/GRDB.swift