[Java] Generate put CharSequence methods for ASCII encoded fields

[zyulyaev]

This PR allows to set ASCII encoded char array fields with any class implementing CharSequence, not only with Strings. It helps writing GC-free code without handling byte arrays. One for example can pass StringBuilder as an argument which can be shared between invocations.

[juddgaddie]

@zyulyaev is there any reason why you did not include something similar on the Decoder side that takes an Appendable such as StringBuilder
eg
public void getMyValue(Appendable val)

[zyulyaev]

@juddgaddie not really, I just added only what I needed.
Regarding Appendable to Decoder, I would rather added a method that returns a wrapper object implementing CharSequence interface, similar to how currently composite decoders work.
eg

private final MyValueWrapper myValueWrapper = new MyValueWrapper();

public CharSequence myValueWrapper() {
    return myValueWrapper;
}

private class MyValueWrapper implements CharSequence {
    public int length() {
        int len;
        for (len = 0; charAt(len) != 0; len++);
        return len;
    }

    public char charAt(int index) {
        final int pos = this.offset + <offset> + index;
        return (char) buffer.getByte(pos);
    }
}