fixes to edify and updater script

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".