Implement [[ fragment literals ]] (#1614)

This feature is referred to as "code/data literals" in ASMotor,
and simply as "literals" in some older assemblers like MIDAS
for the PDP-10. RGBASM already had the "section fragments"
feature for keeping disparate contents together when linked,
so these worked naturally as "fragment literals".

src/asm/parser.y

@@ -113,11 +113,13 @@
 %token YYEOF 0 "end of file"
 %token NEWLINE "end of line"
 %token EOB "end of buffer"
+%token EOL "end of fragment literal"
 
 // General punctuation
 %token COMMA ","
 %token COLON ":" DOUBLE_COLON "::"
 %token LBRACK "[" RBRACK "]"
+%token LBRACKS "[[" RBRACKS "]]"
 %token LPAREN "(" RPAREN ")"
 
 // Arithmetic operators
@@ -381,6 +383,8 @@
 // `scoped_sym_no_anon` exists because anonymous labels usually count as "scoped symbols", but some
 // contexts treat anonymous labels and other labels/symbols differently, e.g. `purge` or `export`.
 %type <std::string> scoped_sym_no_anon
+%type <std::string> fragment_literal
+%type <std::string> fragment_literal_name
 
 // SM83 instruction parameters
 %type <int32_t> reg_r
@@ -455,7 +459,7 @@ line:
 	| line_directive // Directives that manage newlines themselves
 ;
 
-endofline: NEWLINE | EOB;
+endofline: NEWLINE | EOB | EOL;
 
 // For "logistical" reasons, these directives must manage newlines themselves.
 // This is because we need to switch the lexer's mode *after* the newline has been read,
@@ -1308,6 +1312,12 @@ constlist_16bit_entry:
 		    }
 		);
 	}
+	| fragment_literal {
+		Expression expr;
+		expr.makeSymbol($1);
+		expr.checkNBit(16);
+		sect_RelWord(expr, 0);
+	}
 ;
 
 constlist_32bit:
@@ -1358,6 +1368,23 @@ reloc_16bit:
 		$$ = std::move($1);
 		$$.checkNBit(16);
 	}
+	| fragment_literal {
+		$$.makeSymbol($1);
+	}
+;
+
+fragment_literal:
+	LBRACKS fragment_literal_name asm_file RBRACKS {
+		sect_PopSection();
+		$$ = std::move($2);
+	}
+;
+
+fragment_literal_name:
+	%empty {
+		$$ = sect_PushSectionFragmentLiteral();
+		sym_AddLabel($$);
+	}
 ;
 
 relocexpr: