Add support for unions

Unions allow multiple memory allocations (using ds, etc.) to share the
same space in memory.

This allows games to use the same memory for different purposes,
depending on their state.

This also adds documentation on how to use the new UNION, NEXTU, and
ENDU keywords.

src/asm/rgbasm.5

@@ -612,6 +612,41 @@ You can also include only part of a file with
 The example below includes 256 bytes from data.bin starting from byte 78.
 .Pp
 .Dl INCBIN \[dq]data.bin\[dq],78,256
+.Ss Unions
+Unions allow multiple memory allocations to share the same space in memory,
+like unions in C.
+This allows you to easily reuse memory for different purposes, depending on
+the game's state.
+.Pp
+You create unions using the
+.Ic UNION ,
+.Ic NEXTU
+and
+.Ic ENDU
+keywords.
+.Ic NEXTU
+lets you create a new block of allocations, and you may use it as many times
+within a union as necessary.
+.Pp
+.Bd -literal -offset indent
+UNION
+Name: ds 8
+Nickname: ds 8
+NEXTU
+Health: dw
+Something: ds 3
+Lives: db
+NEXTU
+Temporary: ds 19
+ENDU
+.Ed
+.Pp
+This union will use up 19 bytes, as this is the size of the largest block
+(the last one, containing 'Temporary').
+Of course, as 'Name', 'Health', and 'Temporary' all point to the same memory
+locations, writes to any one of these will affect values read from the others.
+.Pp
+Unions may be used in any section, but code and data may not be included.
 .Sh THE MACRO LANGUAGE
 .Pp
 .Ss Printing things during assembly