[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/s1.tolk

@@ -0,0 +1,61 @@
+get ascii_slice(): slice {
+  return"string";
+}
+
+get raw_slice(): slice {
+  return "abcdef"s;
+}
+
+get addr_slice(): slice {
+  return "Ef8zMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzM0vF"a;
+}
+
+get string_hex(): int {
+  return "ABCDEFGHIJKLMNOPQRSTUVWXYZ012345"u;
+}
+
+get fun string_minihash(): int {    // 'get' and 'get fun' both possible
+  return "transfer(slice, int)"h;
+}
+
+get fun string_maxihash(): int {
+  return "transfer(slice, int)"H;
+}
+
+get fun string_crc32(): int {
+  return "transfer(slice, int)"c;
+}
+
+@pure
+fun newc(): builder
+asm "NEWC";
+fun endcs(b: builder): slice
+asm "ENDC" "CTOS";
+@pure
+fun sdeq(s1: slice, s2: slice): int
+asm "SDEQ";
+
+fun main() {
+  var s_ascii: slice = ascii_slice();
+  var s_raw: slice = raw_slice();
+  var s_addr: slice = addr_slice();
+  var i_hex: int = string_hex();
+  var i_mini: int = string_minihash();
+  var i_maxi: int = string_maxihash();
+  var i_crc: int = string_crc32();
+  assert(sdeq(s_ascii, newc().store_uint(0x737472696E67, 12 * 4).endcs())) throw 101;
+  assert(sdeq(s_raw, newc().store_uint(0xABCDEF, 6 * 4).endcs())) throw 102;
+  assert(sdeq(s_addr, newc().store_uint(4, 3).store_int(-1, 8)
+    .store_uint(0x3333333333333333333333333333333333333333333333333333333333333333, 256).endcs()), 103);
+  assert(i_hex == 0x4142434445464748494A4B4C4D4E4F505152535455565758595A303132333435) throw 104;
+  assert(i_mini == 0x7a62e8a8) throw 105;
+  assert(i_maxi == 0x7a62e8a8ebac41bd6de16c65e7be363bc2d2cbc6a0873778dead4795c13db979) throw 106;
+  assert(i_crc == 2235694568) throw 107;
+  return 0;
+}
+
+/**
+@testcase | 0 | | 0
+
+@code_hash 13830542019509784148027107880226447201604257839069192762244575629978154217223
+*/