[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/w2.tolk

@@ -0,0 +1,34 @@
+@method_id(101)
+fun test1(cs: slice) {
+    return cs~load_uint(8)+cs~load_uint(8)+cs~load_uint(8)+cs~load_uint(8);
+}
+
+@method_id(102)
+fun test2(cs: slice) {
+  var (x0, x1, x2, x3, x4, x5, x6, x7, x8, x9, x10,
+       x11, x12, x13, x14, x15, x16, x17, x18, x19) = f(cs);
+  return x0 + x1 + x2 + x3 + x4 + x5 + x6 + x7 + x8 + x9
+       + x10+ x11+ x12+ x13+ x14+ x15+ x16+ x17+ x18+ x19;
+}
+
+fun main(cs: slice) {
+    return (cs~load_uint(8), cs~load_uint(8), cs~load_uint(8), cs~load_uint(8));
+}
+
+fun f(cs: slice) {
+    return (cs~load_uint(8), cs~load_uint(8), cs~load_uint(8), cs~load_uint(8),
+        cs~load_uint(8), cs~load_uint(8), cs~load_uint(8), cs~load_uint(8),
+        cs~load_uint(8), cs~load_uint(8), cs~load_uint(8), cs~load_uint(8),
+        cs~load_uint(8), cs~load_uint(8), cs~load_uint(8), cs~load_uint(8),
+        cs~load_uint(8), cs~load_uint(8), cs~load_uint(8), cs~load_uint(8));
+}
+
+
+/**
+    method_id   | in | out
+@testcase | 102 | x{000102030405060708090a0b0c0d0e0f10111213} | 190
+@testcase | 101 | x{000102030405060708090a0b0c0d0e0f10111213} | 6
+@testcase | 0   | x{000102030405060708090a0b0c0d0e0f10111213} | 0 1 2 3
+
+@code_hash 58474889199998908444151060994149070836199913191952040273624197630531731101157
+*/