[Tolk] v0.6 syntax: fun, import, var, types on the right, etc.

Lots of changes, actually. Most noticeable are:
- traditional //comments
- #include -> import
- a rule "import what you use"
- ~ found -> !found (for -1/0)
- null() -> null
- is_null?(v) -> v == null
- throw is a keyword
- catch with swapped arguments
- throw_if, throw_unless -> assert
- do until -> do while
- elseif -> else if
- drop ifnot, elseifnot
- drop rarely used operators

A testing framework also appears here. All tests existed earlier,
but due to significant syntax changes, their history is useless.

tolk-tester/tests/imports/invalid-no-import.tolk

@@ -0,0 +1,4 @@
+fun demoOfInvalid(): (int) {
+    var f = someAdd;
+    return f(1, 2);
+}

tolk-tester/tests/imports/some-math.tolk

@@ -0,0 +1,3 @@
+fun someAdd(a: int, b: int): int {
+    return a + b + 0;
+}