Unicode step by step

[Leopold Toetsch]

1) Allow usage of installed libicu. Needs some config support to locate 
the necessary includes. Libraries should be found automatically.

2) String PBC layout. The internal string type has changed. This 
currently breaks native_pbc tests (that have strings) as well as some 
"parrot xx.pbc" tests related to strings. The layout seems to depend 
somehow on the supported Unicode levels (or not). So before fixing the 
PBC issues, I'd just have a statememt: parrot_string_t looks such and 
such or of course as is now.

There is of course still the question: Should we really have ICU in the 
tree. This needs tracking updates and patching (again) to make it build 
and so on.

Thanks,
leo