Add "fill until" support

Add "fill until" support

This patch adds an item to add bytes until some offset is reached:

    89 89 aa ff cc dd +0x3c {ICITTE:8}

Result:

    89 89 aa ff cc dd 00 00  00 00 00 00 00 00 00 00  ┆ ••••••••••••••••
    00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ┆ ••••••••••••••••
    00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ┆ ••••••••••••••••
    00 00 00 00 00 00 00 00  00 00 00 00 3c           ┆ ••••••••••••<

The item forms are:

    +EXPR
    +EXPR~PAD

where EXPR is a positive constant integer, a name, or an expression
between `{` and `}`, and PAD is a positive constant integer padding byte
value (0 by default).

The item `+EXPR~PAD` is functionally equivalent to
`PAD * {(EXPR) - ICITTE}`: it's just an alias for what I think is a
common operation.

For example, you might only want to write the first few fields of some
header:

    {le}
    "FRMT"          # Magic
    {0x7855:32}     # Total pixel count
    {data_beg:32}   # Data beginning pointer
    +0x80

    <data_beg>
    "the pixels"    # Here they are

Result:

    46 52 4d 54 55 78 00 00  80 00 00 00 00 00 00 00  ┆ FRMTUx••••••••••
    00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ┆ ••••••••••••••••
    00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ┆ ••••••••••••••••
    00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ┆ ••••••••••••••••
    00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ┆ ••••••••••••••••
    00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ┆ ••••••••••••••••
    00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ┆ ••••••••••••••••
    00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ┆ ••••••••••••••••
    74 68 65 20 70 69 78 65  6c 73                    ┆ the pixels

Change-Id: Ibd3a2bd243c15e1d4f9a9d5b5aa0076e19f9f9f7
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>