bench: Lower the default inputs on many shootout benchmarks

Make them run a bit faster during normal testing

src/test/bench/shootout-ackermann.rs

@@ -18,7 +18,7 @@ fn main(args: [str]) {
     let n = if vec::len(args) == 2u {
         int::from_str(args[1])
     } else {
-        11
+        8
     };
     std::io::println(#fmt("Ack(3,%d): %d\n", n, ack(3, n)));
 }

src/test/bench/shootout-binarytrees.rs

@@ -23,7 +23,7 @@ fn main(args: [str]) {
     let n = if vec::len(args) == 2u {
         int::from_str(args[1])
     } else {
-        15
+        8
     };
     let min_depth = 4;
     let max_depth;

src/test/bench/shootout-fannkuchredux.rs

@@ -62,7 +62,7 @@ fn main(args: [str]) {
     let n = if vec::len(args) == 2u {
         int::from_str(args[1])
     } else {
-        10
+        8
     };
     std::io::println(#fmt("Pfannkuchen(%d) = %d", n, fannkuch(n)));
 }

src/test/bench/shootout-fibo.rs

@@ -14,7 +14,7 @@ fn main(args: [str]) {
     let n = if vec::len(args) == 2u {
         int::from_str(args[1])
     } else {
-        40
+        30
     };
     std::io::println(#fmt("%d\n", fib(n)));
 }

src/test/bench/shootout-nbody.rs

@@ -17,7 +17,7 @@ fn main(args: [str]) {
     let n = if vec::len(args) == 2u {
         int::from_str(args[1])
     } else {
-        1000000
+        100000
     };
     let bodies: [Body::props] = NBodySystem::MakeNBodySystem();
     std::io::println(#fmt("%f", NBodySystem::energy(bodies)));