Locale-sensitive operations using U16String and common locale enumeration

[kugelbltz]

This PR works with PR #2773. Operations are prefixed with uni_

Case modification

uni_to_upper, uni_to_lower and uni_to_title operations using ICU operations.

String sorting

• uni_compare_to : unicode and locale aware string comparison using ICU's collator structure
• Array[String] sorter (StrSorter)

Formatting

Locale aware time/number to String formatting using ICU's number and time formatter structures

Locale

• The default locale is now a global variable (default_locale)
• Added some other common locales in Sys (could be unnecessary)

[Delja]

If you define the same property in two places, it might be suitable to move it in a shared class.

[Morriar]

Also: is it the right place to define this?

[Delja]

Second definition.

[Morriar]

Can we have tests and documentation please?

[Morriar]

Maybe this should be in an object itself added to Sys as to not clutter it more than it already is?

class Locales
	fun chinese_locale: String do return "zh"
	fun english_locale: String do return "en"
	# ...
end

redef class Sys
	fun locales do return new Locales
end

@privat WDYT?

[Morriar]

Also: is it the right place to define this?