Replace uses of Layout.vector with (Layout.tuple o Vector.toList).

Preparing to change Layout.vector to print SML/NJ-style vector
literals.
Change Layout.vector to print SML/NJ-style vector literals.
Change Layout.array to print "#![1,2,3]" style array literals.
Add `val String.implodeV : char vector -> t`.
Update the WordXVector.layout function.

Update the WordXVector.layout function.  If the WordXVector's
elementSize is WordX.word8 and more than 90% of the characters satisfy
Char.isPrint, then display the WordXVector as an SML string constant
(with non-printable characters SML-escaped).  Otherwise, display the
WordXVector as an SML/NJ-style "#[0x0, 0xF]" vector literal.

The layout of large vector literals is not ideal, as the Layout
library (via the internal `sequence` function) with either place all
of the elemnts on the same line or all of the elements on separate
lines, whereas a "paragraph-fill" style would be preferable for large
vector literals.
Replace uses of (Layout.tuple o Vector.toList) with Vector.layout.
Update the WordXVector.layout function.

Update the WordXVector.layout function.  If the WordXVector's
elementSize is WordX.word8 and more than 90% of the characters satisfy
Char.isGraph orelse Char.isSpace, then display the WordXVector as an
SML string constant (with non-printable characters SML-escaped).
Update the WordXVector.layout function.

Update the WordXVector.layout function.  If the WordXVector is not a
reasonable string constant, then display the WordXVector as an
SML/NJ-style "#[0x0, 0xF]" vector literal.  Attempt to obtain a
"paragraph-fill" style by repeatedly packing adjacent pairs of
elements with Layout.mayAlign.
Add `val Layout.fill : t list -> t`.
Update WordX.toString to display "0x" prefix.