func (c info) isLetter() bool {
	ccc := c.cccVal()
	if ccc == cccZero {
		return !c.isCaseIgnorable()
	}
	return ccc != cccBreak
}
isLetter returns whether the rune is of break type ALetter, Hebrew_Letter,
Numeric, ExtendNumLet, or Extend.
package cases
func (c info) cccVal() info {
	if c&exceptionBit != 0 {
		return info(exceptions[c>>exceptionShift]) & cccMask
	}
	return c & cccMask
}
func (c info) cccType() info {
	ccc := c.cccVal()
	if ccc <= cccZero {
		return cccZero
	}
	return ccc
}
func (c info) isBreak() bool {
	return c.cccVal() == cccBreak
}