Reduce stack size of DecimalSymbolsV1

[Manishearth]

Fixes #4437

This doesn't work yet: there's no easy way to get Bake impls for custom VarULE types since from_byte_slice_unchecked can't be called in const contexts (since you can't do trait stuff in non-const contexts: in theory this function is very const-capable)

I chose to use make_varule but VarTupleNULE would work as well (and plus we can give it the relevant Bake impl if we want). I wrote this PR to encapsulate things so that swapping the internals is easier

I think I'll make this work by implementing ZeroCow: #5561

[Manishearth]

Updated to use #5809 and #5808

[sffc]

Good to here but there's still one comment open

[sffc]

---- datagen::test_markers_for_bin stdout ----
thread 'datagen::test_markers_for_bin' panicked at components/icu/src/datagen.rs:96:5:
assertion `left == right` failed
  left: [datetime/names/dayperiods@1, datetime/names/gregory/months@1, datetime/names/gregory/years@1, datetime/names/weekdays@1, datetime/patterns/glue@1, datetime/patterns/gregory/skeleton@1, datetime/patterns/time_skeleton@1, datetime/week_data@2]
 right: [datetime/names/dayperiods@1, datetime/names/gregory/months@1, datetime/names/gregory/years@1, datetime/names/weekdays@1, datetime/patterns/glue@1, datetime/patterns/gregory/skeleton@1, datetime/patterns/time_skeleton@1, datetime/week_data@2, decimal/symbols@2]