fix: LHS of pointer assignments can only be pointer arithmetic operations

[GGBOIH250]

In compile_assignment the ltype is an expression and can be changed to ltype = compile_arithmetic() as we only have pointer arithmetic or identifier dereferencing on the left hand side of assignments in C*. The default implementation from selfie compiles this with only warnings and no errors:

int main() {
    int *x;
    int *y;

    *(x == y) = 12;

    return 0;
}

Although running this results in a segfault, I find that compiling it successfully is also not ideal. Changing it to ltype = compile_arithmetic() in compile_assignment, the self compilation doesn't break under make self and make self-self and it returns syntax error when above code is now tried to compile.

[ckirsch]

@GGBOIH250 thanks a lot!