Lex $foo as a single token, instead of using '$' IDENT

Previously, variable/symbols were parsed as the combination of two
tokens: '$' IDENT

This meant that using a keyword as variable name (e.g. $then, $label)
did not work.
Attempts were made to allow $keyword to work by adding some '$' Keyword
rules in the parser, but this did not allow $keyword in all places:

  jq --arg label foo -n '$label'  # ok

  jq -n '"foo" as $label | .'     # error

Treating $foo as a single token is much simpler, in my opinion.

This patch also changes how LOC is lexed: "$__loc__" instead of as
"__loc__" that gets combined with '$' in the parser.

This patch also disallows having spaces after '$' when recalling a
variable  `$ foo'  since that was probably just an unintentional side
effect of the implementation, and it was not documented.

Fixes #2675

src/lexer.h

@@ -285,7 +285,6 @@ typedef int16_t flex_int16_t;
 typedef uint16_t flex_uint16_t;
 typedef int32_t flex_int32_t;
 typedef uint32_t flex_uint32_t;
-typedef uint64_t flex_uint64_t;
 #else
 typedef signed char flex_int8_t;
 typedef short int flex_int16_t;
@@ -399,7 +398,7 @@ struct yy_buffer_state
 	/* Number of characters read into yy_ch_buf, not including EOB
 	 * characters.
 	 */
-	yy_size_t yy_n_chars;
+	int yy_n_chars;
 
 	/* Whether we "own" the buffer - i.e., we know we created it,
 	 * and can realloc() it to grow it, and should free() it to
@@ -443,7 +442,7 @@ void yypop_buffer_state ( yyscan_t yyscanner );
 
 YY_BUFFER_STATE yy_scan_buffer ( char *base, yy_size_t size , yyscan_t yyscanner );
 YY_BUFFER_STATE yy_scan_string ( const char *yy_str , yyscan_t yyscanner );
-YY_BUFFER_STATE yy_scan_bytes ( const char *bytes, yy_size_t len , yyscan_t yyscanner );
+YY_BUFFER_STATE yy_scan_bytes ( const char *bytes, int len , yyscan_t yyscanner );
 
 void *yyalloc ( yy_size_t , yyscan_t yyscanner );
 void *yyrealloc ( void *, yy_size_t , yyscan_t yyscanner );
@@ -501,7 +500,7 @@ FILE *yyget_out ( yyscan_t yyscanner );
 
 void yyset_out  ( FILE * _out_str , yyscan_t yyscanner );
 
-			yy_size_t yyget_leng ( yyscan_t yyscanner );
+			int yyget_leng ( yyscan_t yyscanner );
 
 char *yyget_text ( yyscan_t yyscanner );
 
@@ -732,9 +731,9 @@ extern int yylex \
 #undef yyTABLES_NAME
 #endif
 
-#line 130 "src/lexer.l"
+#line 131 "src/lexer.l"
 
 
-#line 738 "src/lexer.h"
+#line 737 "src/lexer.h"
 #undef jq_yyIN_HEADER
 #endif /* jq_yyHEADER_H */

src/lexer.l

@@ -60,7 +60,7 @@ struct lexer_param;
 "catch" { return CATCH; }
 "label" { return LABEL; }
 "break" { return BREAK; }
-"__loc__" { return LOC; }
+"$__loc__" { return LOC; }
 "|=" { return SETPIPE; }
 "+=" { return SETPLUS; }
 "-=" { return SETMINUS; }
@@ -122,6 +122,7 @@ struct lexer_param;
 
 ([a-zA-Z_][a-zA-Z_0-9]*::)*[a-zA-Z_][a-zA-Z_0-9]*  { yylval->literal = jv_string(yytext); return IDENT;}
 \.[a-zA-Z_][a-zA-Z_0-9]*  { yylval->literal = jv_string(yytext+1); return FIELD;}
+\$[a-zA-Z_][a-zA-Z_0-9]*  { yylval->literal = jv_string(yytext+1); return VARIABLE;}
 
 [ \n\t]+  {}