Parser: corpus parse rate 81% -> 90%

A bare procedure *type* parameter such as 'proc: (info: *Info, data: T)' has no
arrow, directive or body after the parentheses, so isProcHeaderAhead now accepts
any parenthesised group that contains a ':' or '$' at depth 1.

Also: compound-assignment operator overloads, 'for <=REVERSE *=BY_POINTER',
backticked loop variables and returns, 'cast(T, value)', postfix 'x.(T)' casts,
'{}' as the zero value, '#asm AVX, AVX2 { … }', 'push_context,defer_pop', and
'#string,\%' here-strings (used in how_to/018_print_functions.jai, which the
shipped Jai_Lexer module itself rejects).

A directive prefixing a declaration no longer takes a bare operand: '#add_context
deep_copy_depth: int;' had the directive eating the declared name.

Ratchet raised to 640/714.

src/main/kotlin/dev/hgh/jai/parser/JaiParserUtil.kt

@@ -282,11 +282,15 @@ object JaiParserUtil : GeneratedParserUtilBase() {
                 T.RPAREN -> {
                     depth--
                     if (depth == 0) {
+                        // A `name:` or `$T` inside the parentheses can only be a
+                        // parameter list, so the header stands on its own: a bare
+                        // proc *type* like `proc: (info: *Info, data: T)` has no
+                        // arrow, directive or body after it.
+                        if (sawParamMarker) return true
                         val next = b.lookAhead(i + 1)
-                        return when {
-                            next === T.RIGHT_ARROW -> true
-                            next === T.DIRECTIVE -> true
-                            next === T.LBRACE -> empty || sawParamMarker
+                        return when (next) {
+                            T.RIGHT_ARROW, T.DIRECTIVE -> true
+                            T.LBRACE -> empty
                             else -> false
                         }
                     }
@@ -459,6 +463,7 @@ object JaiParserUtil : GeneratedParserUtilBase() {
     ): Boolean {
         var i = 0
         while (true) {
+            if (b.lookAhead(i) === T.BACKTICK) i++
             if (b.lookAhead(i) !== T.IDENT) return false
             i++
             when (b.lookAhead(i)) {