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
+*/

test/asm/readfile-binary.out.bin

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+!#%')"$&(0 <20>!<21><>

test/asm/readfile-nonexist.asm

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+def s equs readfile("readfile-nonexist.inc")
+assert strlen(#s) == 0

test/asm/readfile-nonexist.err

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+error: readfile-nonexist.asm(1):
+    Error opening READFILE file 'readfile-nonexist.inc': No such file or directory
+Assembly aborted with 1 error!

test/asm/readfile.asm

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+def s equs readfile("readfile.inc")
+println strupr(#s) ++ "!"
+
+redef s equs readfile("readfile.inc", 5)
+println strrpl(#s, "l", "w") ++ "?"

test/asm/readfile.inc

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+hello world

test/asm/readfile.out

@@ -0,0 +1,2 @@
+HELLO WORLD!
+hewwo?