uxn rom metadata proposal

by d6

currently uxn rom files are just the data that will be loaded into the VMs
memory on start up (starting with address 0x100 since the zero page is
skipped). this means that the maximum rom size is 65280 bytes, although
most roms are much smaller since trailing zeros can be left out.

this simplicity is great, but comes with some (possible) downsides:

 - roms aren't identifiable beyond their file name
 - roms don't contain any attribution information, credits, or licenses
 - roms don't contain a version information (rom version or uxn version)
 - roms don't contain any icon or preview information

while it would be nice to add all of these things that would create a major
burden on assembler and emulator authors. i think there's an easier path
forward.

i propose adding four bytes to the start of every rom:

 - the literal 3 bytes "uxn"
 - a fourth mode byte

this proposal just covers mode 0 and mode 1, but in the future we have up to
254 other modes to use (though we might choose to forbid those later to keep
things simple).

the "uxn0" format would be exactly what we have now, just with those four
bytes at the start. assembler authors could choose to only support creating
"uxn0" roms without very much extra effort over what they do now. emulator
authors could easily adapt their current work to read this format. rom files
that lack a "uxn" at the start could continue to work as they do now.

the "uxn1" format would provide some extra metadata:

 - total-size   (2 bytes):  total metadata size (including "uxn1")
 - uxn-version  (2 bytes):  0x0000 for unspecified, 0x0001 for current
 - name-size    (1 byte):   size of the following name string in bytes
 - name         (n bytes):  the name string (ASCII/UTF-8)
 - version-size (1 byte):   size of the version string in bytes
 - version      (n bytes):  the version string (ASCII/UTF-8)
 - author-size  (1 byte):   size of the author string in bytes
 - author       (n bytes):  the author string (ASCII/UTF-8)
 - desc-size    (2 bytes):  size of the description string in bytes (4096 max)
 - desc         (n bytes):  the description string (ASCII/UTF-8)
 - icon-type    (1 byte):   the size and depth of the icon
 - icon-palette (n bytes):  the icon's color theme
 - icon-chr     (n bytes):  the icon's CHR data

the minimal "uxn1" header size (assuming the strings and icon are all empty)
would be 10 bytes (2 + 2 + 1 + 1 + 1 + 2 + 1). emulator implementors could
read total-size and then seek past this metadata to read the rom data.

the icon types would be:

 - 0x00: no icon provided (0-byte icon-palette,    0-byte icon-chr)
 - 0x01: 1-bit 16x16 icon (2-byte icon-palette,   32-byte icon-chr)
 - 0x02: 1-bit 32x32 icon (2-byte icon-palette,  128-byte icon-chr)
 - 0x03: 1-bit 64x64 icon (2-byte icon-palette,  512-byte icon-chr)
 - 0x81: 2-bit 16x16 icon (6-byte icon-palette,   64-byte icon-chr)
 - 0x82: 2-bit 32x32 icon (6-byte icon-palette,  256-byte icon-chr)
 - 0x83: 2-bit 64x64 icon (6-byte icon-palette, 1024-byte icon-chr)