v5: More types, generics

[guregu]

• Now a Go module under the path github.com/guregu/null/v5
• Added missing types from database/sql: Int32, Int16, Byte
• Added generic Value[T] embedding sql.Null[T]
  • This doesn't support TextMarshaler because there's no generic way to support all types (e.g. structs). We could support some types but I'm not sure if that's useful.

Behavior from v4 should be unchanged, but the canonical import path is different so I figure it's best to bump it.

Closes #67