Fix the hacky solution for adjusting FlexSym system SIDs

[popematt]

A hacky solution was introduced in #944. It works for now, but it will break if the Ion 1.1 system symbol table gets significantly larger.

ion-java/src/main/java/com/amazon/ion/impl/IonReaderContinuableCoreBinary.java

Lines 2457 to 2467 in c5562bf

	// FIXME: This is a hack that works as long as our system symbol table doesn't grow to
	// more than ~95 symbols. We need this hack because when we have to read the FixedInt,
	// we don't know whether it's a tagless FlexSym or a Regular value.
	// Possible solutions include:
	// * changing the spec so that FlexSym System SIDs line up with the regular System SIDs
	// * Introducing a dummy IonTypeID that indicates that we need to add the bias
	// * Update IonCursorBinary.slowSkipFlexSym_1_1() to put the id into valueMarker.endIndex,
	// though that seems to have its own problems.
	if (id >= FLEX_SYM_SYSTEM_SYMBOL_OFFSET) {
	id = id - FLEX_SYM_SYSTEM_SYMBOL_OFFSET;
	}

Part of the fix may involve solving this problem:

ion-java/src/main/java/com/amazon/ion/impl/IonCursorBinary.java

Lines 1737 to 1741 in c5562bf

	// FIXME: See if we can set the SID in the endIndex here without causing the slow reader to get confused
	// about where the end of the value is for tagless symbols.
	// I.e. use setSystemSymbolMarker(markerToSet, (byte)(specialByte - FLEX_SYM_SYSTEM_SYMBOL_OFFSET));
	markerToSet.typeId = SYSTEM_SYMBOL_VALUE;
	markerToSet.startIndex = peekIndex - 1;