Stop storing rules from 1 to nrules + 1.

* src/LR0.c, src/closure.c, src/derives.c, src/gram.c, src/lalr.c
* src/nullable.c, src/output.c, src/print.c, src/reader.c
* src/reduce.c: Allocate and free from &rules[0], not &rules[1].
Iterate from 0 to nrules.
Use rule_number_as_item_number and item_number_as_rule_number.
Adjust to `derive' now containing possibly 0.
* src/gram.h (rule_number_as_item_number, item_number_as_rule_number):
Handle the `- 1' part in rule numbers from/to item numbers.
* src/conflicts.c (log_resolution): Fix the message which reversed
shift and reduce.
* src/output.c (action_row): Initialize default_rule to -1.
(token_actions): Adjust.
* tests/sets.at (Nullable, Firsts): Fix the previously bogus
expected output.
* tests/conflicts.at (Resolved SR Conflicts): Likewise.

tests/conflicts.at

@@ -326,7 +326,7 @@ state 5
     1    | exp OP exp .  [$, OP]
 
     $default  reduce using rule 1 (exp)
-    Conflict between rule 2 and token OP resolved as shift (%left OP).
+    Conflict between rule 1 and token OP resolved as reduce (%left OP).
 ]])
 
 AT_CLEANUP

tests/sets.at

@@ -98,12 +98,12 @@ FIRSTS
 		e
 FDERIVES
 	$axiom derives
-		  1  e $
-		  2  'e'
-		  3  /* empty */
+		  0  e $
+		  1  'e'
+		  2  /* empty */
 	e derives
-		  2  'e'
-		  3  /* empty */
+		  1  'e'
+		  2  /* empty */
 ]])
 
 AT_CLEANUP
@@ -233,22 +233,22 @@ FIRSTS
 		exp
 FDERIVES
 	$axiom derives
-		  1  exp $
-		  2  exp '<' exp
-		  3  exp '>' exp
-		  4  exp '+' exp
-		  5  exp '-' exp
-		  6  exp '^' exp
-		  7  exp '=' exp
-		  8  "exp"
+		  0  exp $
+		  1  exp '<' exp
+		  2  exp '>' exp
+		  3  exp '+' exp
+		  4  exp '-' exp
+		  5  exp '^' exp
+		  6  exp '=' exp
+		  7  "exp"
 	exp derives
-		  2  exp '<' exp
-		  3  exp '>' exp
-		  4  exp '+' exp
-		  5  exp '-' exp
-		  6  exp '^' exp
-		  7  exp '=' exp
-		  8  "exp"
+		  1  exp '<' exp
+		  2  exp '>' exp
+		  3  exp '+' exp
+		  4  exp '-' exp
+		  5  exp '^' exp
+		  6  exp '=' exp
+		  7  "exp"
 ]])
 
 AT_CLEANUP