Helper function to build valid UTF-16 Java string from UFT-8

Handle code point in the range U+10000..U+1FFFFF and generate matching
surrogate-pair.

Change-Id: Ib225f299e1b6e09aad660a0c064a20bc93bc88f9
Update list of AUTHORS

Change-Id: I5a55d91d3407be6b84e850afe80355d86171789d
Hint compiler about maximal length of digest when generating HEX of digest

Static analyzer in GCC 4.9+ (e.g. NDK 10b on Android) emits warning about
potential out of bounds access to digest[]. Make it explicit that this never
happens.

Change-Id: Id71b9617dfc1dd4a37e1b17a3ffe1b2a69845367
CONST-ify write API for Ychannel

Change-Id: I1167e136331bb8761d1ce396a60d07ce8041ef7f