Disable implicit static string concatenation. Correct concern tests.

Before (and in Erlang) `"Hello" " World"` would be compiled into `"Hello World"`.
This is problematic for a separator-less language as seen below and in https://github.com/fenollp/kju/pull/22.

Change
```erlang
f({connect,_,To,_,_} = Signal, To, 1+2 bxor 3 = 0) =
    S = ("prefixed string") # That's embarrassing
    "prefix"++Str = S
```
to
```erlang
f({connect,_,To,_,_} = Signal, To, 1+2 bxor 3 = 0) =
    S = "prefixed string"
    "prefix"++Str = S
```

String concatenation at compile time can still be done, although explicitly: using `++`.
That's some more characters to write to accomplish the same thing, but it saves the language some embarrassing bracing!