[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/special-fun-names.tolk

@@ -0,0 +1,24 @@
+fun onInternalMessage() { return 0; }
+fun onExternalMessage() { return -1; }
+fun onRunTickTock() { return -2; }
+fun onSplitPrepare() { return -3; }
+fun onSplitInstall() { return -4; }
+
+/**
+@experimental_options remove-unused-functions
+
+@testcase | 0  | | 0
+@testcase | -1 | | -1
+@testcase | -2 | | -2
+@testcase | -3 | | -3
+@testcase | -4 | | -4
+
+@fif_codegen
+"""
+  0 DECLMETHOD onInternalMessage
+  -1 DECLMETHOD onExternalMessage
+  -2 DECLMETHOD onRunTickTock
+  -3 DECLMETHOD onSplitPrepare
+  -4 DECLMETHOD onSplitInstall
+"""
+ */