Fast String Appends

[gfwilliams]

The first JSV_STRING in a chain could actually store only 8 characters (but a link to the last JSV_STRINGEXT in that chain). It'd allow us to do much faster appends which would help out in a number of areas.

[gfwilliams]

Potentially we could store a link to the last accessed STRINGEXT as well as its character index. This would not only make appends faster, but also accesses (including ArrayBuffer accesses).

[gfwilliams]

Not going to do this now - of very limited use now with 'flat strings' and ArrayBuffers