turn off slice bounds checking in release mode

This change removes bounds checking for Slices, which represents most
indexable storage in the emulator. Removing these checks makes the
emulator less safe, without a doubt. However, it amounts a roughly 5%
improvement in the games tested. Most of the checks today occur in
fixed-length arrays where the index cannot be out of bounds, so I'm
chosing to make this tradeoff.

src/crab.cr

@@ -1,3 +1,19 @@
+{% if flag?(:release) %}
+  # Disables bounds checking in release mode.
+
+  struct Slice(T)
+    @[AlwaysInline]
+    def []=(index : Int, value : T) : T
+      @pointer[index] = value
+    end
+
+    @[AlwaysInline]
+    def [](index : Int) : T
+      @pointer[index]
+    end
+  end
+{% end %}
+
 require "colorize"
 require "option_parser"