Optimize #with

This PR is inspired by #56, and
assumes that code will be merged, so uses it in the benchmarks here:
https://gist.github.com/ms-ati/fa8002ef8a0ce00716e9aa6510d3d4d9

It is common in our code, as in any idiomatic code using value objects
in loops or pipelines, to call `#with` many times, returning a new immutable
object each time with 1 or more fields replaced with new values.

The optimizations in this PR eliminate a number of extra Hash and Array
instantiations that were occurring each time, in favor of iterating only over
the constant `VALUE_ATTRS` array and doing key lookups in the given
Hash parameter in the hot paths.

Per the gist above, this increases ips (iterations per second) 2.29x, from
335.9 to 769.6 on my machine.

lib/values.rb

@@ -38,13 +38,15 @@ def self.new(*fields, &block)
       const_set :VALUE_ATTRS, fields
 
       def self.with(hash)
-        unexpected_keys = hash.keys - self::VALUE_ATTRS
-        if unexpected_keys.any?
+        num_recognized_keys = self::VALUE_ATTRS.count { |field| hash.key?(field) }
+
+        if num_recognized_keys != hash.size
+          unexpected_keys = hash.keys - self::VALUE_ATTRS
           raise ArgumentError.new("Unexpected hash keys: #{unexpected_keys}")
         end
 
-        missing_keys = self::VALUE_ATTRS - hash.keys
-        if missing_keys.any?
+        if num_recognized_keys != self::VALUE_ATTRS.size
+          missing_keys = self::VALUE_ATTRS - hash.keys
           raise ArgumentError.new("Missing hash keys: #{missing_keys} (got keys #{hash.keys})")
         end
 
@@ -83,9 +85,22 @@ def pretty_print(q)
         end
       end
 
+      # Optimized to avoid intermediate Hash instantiations.
       def with(hash = {})
         return self if hash.empty?
-        self.class.with(to_h.merge(hash))
+
+        num_recognized_keys = self.class::VALUE_ATTRS.count { |field| hash.key?(field) }
+
+        if num_recognized_keys != hash.size
+          unexpected_keys = hash.keys - self.class::VALUE_ATTRS
+          raise ArgumentError.new("Unexpected hash keys: #{unexpected_keys}")
+        end
+
+        args = self.class::VALUE_ATTRS.map do |field|
+          hash.key?(field) ? hash[field] : send(field)
+        end
+
+        self.class.new(*args)
       end
 
       def to_h