Simple obfuscation for large literals

[DominicBreuker]

Changes literal obfuscation such that literals of any size will be obfuscated, but beyond maxSize we only use the simple obfuscator. This one seems to apply one of the AND, OR or XOR operators byte-wise and should be safe to use.

The test for literals is changed a bit to verify that obfuscation is applied. The code written to the extra_literals.go file by the test helper now creates an init function with Printf statements inside. This way Go does not optimize the literals away when we compile. We also append a unique string to all literals so that we can test that an unobfuscated build contains this string while an obfuscated build does not.

[mvdan]

Thanks for the patch! Just some minor stuff.

FYI @lu4p if you have any thoughts.

[DominicBreuker]

attempt no. 2
test changed such that the -o out is gone. also added some comments to the test helper regarding the compiler optimization.
as for the code in extra_literals.go: agree that plain println would have been better, but I noticed that it might be even better to not print anything at all since failing tests otherwise mess up the terminal. thus I switched to an init function that appends literals to a global variable. like this:

var x = ""

func init() {
    x += "first_string"
    x += "second_string"
    ...
}

[mvdan]

Thanks! Merging on green.