A few more changes to edify:
- fix write_raw_image(); my last change neglected to close the write
context, so the written image was corrupt.
- each expression tracks the span of the source code from which it
was compiled, so that assert()'s error message can include the
source of the expression that failed.
- the 'cookie' argument to each Function is replaced with a State
object, which contains the cookie, the source script (for use with
the above spans), and the current error message (replacing the
global variables that were used for this purpose).
- in the recovery image, a new command "ui_print" can be sent back
through the command pipe to cause text to appear on the screen.
Add a new ui_print() function to print things from scripts.
Rename existing "print" function to "stdout".
