Implement READFILE function (#1759)

test/asm/readfile-binary.asm

@@ -0,0 +1,24 @@
+section "tilemap", rom0
+/*
+input:
+$20
+$01 $03 $05 $07 $09
+$02 $04 $06 $08 $10
+$00 $de $01 $df $80
+5
+*/
+def tilemap equs readfile("readfile-binary.inc.bin")
+def area = bytelen(#tilemap) - 2
+def offset = strbyte(#tilemap, 0)
+def width = strbyte(#tilemap, area + 1)
+db width, area / width
+for idx, area
+  db strbyte(#tilemap, idx + 1) + offset
+endr
+/*
+output:
+5, 3
+$21 $23 $25 $27 $29
+$22 $24 $26 $28 $30
+$20 $fe $21 $ff $a0
+*/