RFR: JDK-8021560,(str) String constructors that take ByteBuffer

[Xueming Shen]

Hi,

Please help review the proposal to add following constructors and 
methods in String
class to take ByteBuffer as the input and output data buffer.

public String(ByteBuffer bytes, Charset cs);
public String(ByteBuffer bytes, String csname);
public int getBytes(byte[] dst, int offset, Charset cs);
public int getBytes(byte[] dst, int offset, String csname);
public int getBytes(ByteBuffer bytes, Charset cs);
public int getBytes(ByteBuffer bytes, Charset csn);

This is based on the change proposed by Richard in jdk9 time frame [1]. 
But since
we were fully occupied by compact string support work back then, this 
one did
not make into the top jdk9 priority list.

issue: https://bugs.openjdk.java.net/browse/JDK-8021560
webrev: http://cr.openjdk.java.net/~sherman/8021560/webrev

The implementation in StringCoding class is a little "duplicated" for 
various types
of encoding/decoding variants. There is definitely room to further 
consolidate. But
I would assume it might be better to keep the new code path separated 
from the
existing "fully tuned" methods, for now, to avoid possible performance 
regression
for existing String methods.

Thanks,
Sherman

[1] 
http://mail.openjdk.java.net/pipermail/core-libs-dev/2014-November/029672.html