[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/bit-operators.tolk

@@ -0,0 +1,53 @@
+fun lshift(): int {
+    return (1 << 0) == 1;
+}
+
+fun rshift(): int {
+    return (1 >> 0) == 1;
+}
+
+fun lshift_var(i: int): int {
+    return (1 << i) == 1;
+}
+
+fun rshift_var(i: int): int {
+    return (1 >> i) == 1;
+}
+
+fun main(x: int): int {
+    if (x == 0) {
+        return lshift();
+    } else if (x == 1) {
+        return rshift();
+    } else if (x == 2) {
+        return lshift_var(0);
+    } else if (x == 3) {
+        return rshift_var(0);
+    } else if (x == 4) {
+        return lshift_var(1);
+    } else {
+        return rshift_var(1);
+    }
+}
+
+@method_id(11)
+fun is_claimed(index: int): int {
+    var claim_bit_index: int = index % 256;
+    var mask: int = 1 << claim_bit_index;
+    return (255 & mask) == mask;
+}
+
+
+/**
+    method_id | in  | out
+@testcase | 0  | 0   | -1
+@testcase | 0  | 1   | -1
+@testcase | 0  | 2   | -1
+@testcase | 0  | 3   | -1
+@testcase | 0  | 4   |  0
+@testcase | 0  | 5   |  0
+@testcase | 11 | 0   | -1
+@testcase | 11 | 1   | -1
+@testcase | 11 | 256 | -1
+@testcase | 11 | 8   |  0
+*/