Fix parser translator crash for certain octal escapes

lib/prism/polyfill/append_as_bytes.rb

@@ -5,7 +5,10 @@
   String.include(
     Module.new {
       def append_as_bytes(*args)
-        args.each { self.<<(_1.b) } # steep:ignore
+        args.each do |arg|
+          arg = Integer === arg ? [arg].pack("C") : arg.b
+          self.<<(arg) # steep:ignore
+        end
       end
     }
   )

lib/prism/translation/parser/lexer.rb

@@ -725,7 +725,7 @@ def simplify_string?(value, quote)
         def escape_build(value, control, meta)
           value &= 0x9f if control
           value |= 0x80 if meta
-          value.chr
+          value
         end
 
         # Read an escape out of the string scanner, given the control and meta

test/prism/fixtures/strings.txt

@@ -126,6 +126,8 @@ baz
 
 "ち\xE3\x81\xFF"
 
+"\777"
+
 %[abc]
 
 %(abc)