The `Range` type defined by uts46 takes up 20 bytes on a 32-bit platform
and 32 bytes on a 64-bit platform.  This bloat is due to `Mapping` using
a full word-width discriminant for its type and native string slices,
which are unnecessarily large.  The use of native string slices in this
context also require many load-time relocations on some platforms when
idna is used in a static library, another unnecessary cost.

Instead of this scheme, we can define our own string slice type that
indexes into a generated table.  This string slice type enables us to
use a smaller discriminant type for `Mapping`, and therefore reduces the
size of `Range` to 16 bytes on a 32-bit platform and 16 bytes on a
64-bit platform.  For x86-64 Linux, this change reduces total idna size
by ~130K, a significant savings.

Changing the script to use unicode escapes makes things slightly more
readable, enables better eyeball comparisons of the generated tables to
the original mapping table, and makes syntax highlighting in emacs
somewhat faster.