External modes test and minor optimization

[solardiz]

This tests almost all external modes on make check, which we run on CI.

This also adds proper compile-time handling of negative integer constants, which previously emitted the unary - instruction. Unfortunately, the code for this is a hack. It was cleaner not to handle this case specially.

My additional test case (not included here) for this optimization is:

[List.External:Test]
void generate()
{
	if (word[2] == 666) {
		word = 0;
		return;
	}
	word[0] =-1 + 'a' + 1 - -111 -111 - 222 + 222 -111-111+222;
	word[0] -=-22;
	word[0] -= 22;
	word[0]-=-22;
	word[0]-=22;
	word[1] = -1 + 1;
	word[2] = 666;
}

This has a mix of - in different meanings, which the compiler has to distinguish properly. The expected output is a.

[solardiz]

This also adds proper compile-time handling of negative integer constants, which previously emitted the unary - instruction. Unfortunately, the code for this is a hack. It was cleaner not to handle this case specially.

We should be able to drop this hack later with no performance loss if we instead implement generic compile-time evaluation of constant subexpressions.