Support "full" case mapping in Strings: toLower/toUpper

[pbiggar]

toLower and toUpper don't work for strings where the converted string has a different number of EGCs than the original. For example:

String.toUppercase_v1 "fifl" = "FIFL"

See

dark/fsharp-backend/tests/testfiles/string.tests

Line 518 in cf5f36b

// FSTODO: There are two types of unicode case "mapping" (conversion), "simple"

for discussion.

[pbiggar]

I was recently rereading the dotnet GitHub issue here, where they said dotnet only supports simple mapping. We should file a bug for dotnet to support full mapping.

[StachuDotNet]

folded into #5239