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/test/kotlin/dev/hgh/jai/parser/JaiCorpusParserTest.kt

@@ -106,7 +106,7 @@ class JaiCorpusParserTest : BasePlatformTestCase() {
 
     private companion object {
         /** Ratchet. Raise it as the grammar improves; never lower it. */
-        const val MIN_CLEAN_FILES = 575
+        const val MIN_CLEAN_FILES = 640
         const val REPORTED_ERRORS = 25
         const val REPORTED_KINDS = 20
         const val GOT_WIDTH = 14