feat: add benchmark chart visualization to performance page

Refactor benchmark runner to store results in a nested dict keyed by language name, enabling per-benchmark result aggregation. Add bar chart HTML tables to doc/site/performance.markdown displaying method call, DeltaBlue, and binary trees benchmarks across languages. Include SCSS styles for chart layout, bar coloring, and main content padding.

doc/site/style.scss

@@ -189,6 +189,10 @@ a {
   outline: none;
 }
 
+main {
+  padding-top: 12px;
+}
+
 a:hover {
   color: $blue-dark;
 }
@@ -299,6 +303,38 @@ body.reference {
   }
 }
 
+// Bar charts on the performance page.
+table.chart {
+  width: 100%;
+
+  td, th {
+    line-height: 14px;
+    margin: 0;
+    padding: 0;
+  }
+
+  th {
+    font-size: 14px;
+    text-align: left;
+    width: 100px;
+  }
+
+  .chart-bar {
+    display: inline-block;
+    font: 13px $body;
+    color: $light;
+    background: $blue;
+    border-bottom: solid 1px $blue-dark;
+    text-align: right;
+    border-radius: 2px;
+  }
+
+  .chart-bar.wren {
+    background: mix($blue, $blue-dark, 30%);
+    border-bottom: solid 1px $blue-dark;
+  }
+}
+
 /*
 @media only screen and (max-width: 639px) {
   // Shrink the window padding.