[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/pure-functions.tolk

@@ -0,0 +1,46 @@
+
+@pure
+fun f_pure1(): int {
+    return f_pure2();
+}
+
+@pure
+fun f_pure2(): int {
+    return 2;
+}
+
+@pure
+fun get_contract_data(): (int, int) {
+    var c: cell = get_data();
+    var cs: slice = c.begin_parse();
+    cs~load_bits(32);
+    var value: int = cs~load_uint(16);
+    return (1, value);
+}
+
+fun save_contract_data(value: int) {
+    var b: builder = begin_cell().store_int(1, 32).store_uint(value, 16);
+    set_data(b.end_cell());
+}
+
+@pure
+@method_id(101)
+fun test1(): int {
+    return f_pure1();
+}
+
+@method_id(102)
+fun test2(value: int): int {
+    save_contract_data(value);
+    var (_, restored: auto) = get_contract_data();
+    return restored;
+}
+
+fun main() { return; }
+
+/**
+
+@testcase | 101 |     | 2
+@testcase | 102 |  44 | 44
+
+*/