port UTF8 string manipulation code from mother project

[rdmark]

this is to evaluate and discuss whether the UTF8 code should be ported over from bstrlib; this was functionality that was implemented after this fork was forked off of bstrlib

some time ago I did a brief experiment but porting wasn't entirely trivial due to some drift between the two projects

[rdmark]

these PRs gives us parity with bstrlib, minus the C++ wrappers

• port UTF8 string manipulation modules from bstrlib #142
• add biseqblk, biseqcaselessblk, binsertblk, bjoinblk #143
• port bSGMLEncode function from bstrlib #147