Some Span<Character> cannot be instantiated

[CeylonMigrationBot]

[@tombentley] It's impossible to construct some Span<Character> because the constructor of Span has to compute recursive which requires first.successor and last.predecessor and for Character '\0'..predecessor and '\{#10FFFF}'.successor both throw. So precisely those Spans whose first is '\{#10FFFF}' or whose last is '\0' are affected.

FTR, I don't think this is a huge problem

[Migrated from ceylon/ceylon.language#717]

[CeylonMigrationBot]

[@FroMage] This sounds like a 1.3 issue.