WordXVector updates

Various updates to `structure WordXVector`.  Highlights:
 * Update the `WordXVector.layout` function.  If the `elementSize` is `WordX.word8` and more than 90% of the characters satisfy `Char.isGraph orelse Char.isSpace`, then display as an SML string constant (with non-printable characters SML-escaped).  Otherwise, display as an SML/NJ-style `#[0x0, 0xF]` vector literal.
 * Update initialization of `static struct GC_vectorInit vectorInits[]` constants in runtime.  If the `WordXVector`'s (primitive) `elementSize` is `WordSize.W8`, then emit a C-escaped string constant.  Otherwise, emit a C-array initialization.