fix some logic errors

[froydnj]

While running bundle exec ruby benchmark.rb, I ran into an error:

st-froydnj2:message-format-rb froydnj$ RBENV_VERSION=3.1 bundle exec ruby benchmark.rb
       user     system      total        real
parse simple message  0.623193   0.003719   0.626912 (  0.626966)
format simple message/Users/froydnj/stripe/message-format-rb/lib/message_format.rb:9:in `initialize': wrong number of arguments (given 3, expected 1..2) (ArgumentError)
	from /Users/froydnj/stripe/message-format-rb/lib/message_format.rb:29:in `new'
	from /Users/froydnj/stripe/message-format-rb/lib/message_format.rb:29:in `new'
	from benchmark.rb:15:in `block (2 levels) in <main>'
	from /Users/froydnj/.rbenv/versions/3.1.4/lib/ruby/3.1.0/benchmark.rb:296:in `measure'
	from /Users/froydnj/.rbenv/versions/3.1.4/lib/ruby/3.1.0/benchmark.rb:378:in `item'
	from benchmark.rb:14:in `block in <main>'
	from /Users/froydnj/.rbenv/versions/3.1.4/lib/ruby/3.1.0/benchmark.rb:176:in `benchmark'
	from /Users/froydnj/.rbenv/versions/3.1.4/lib/ruby/3.1.0/benchmark.rb:208:in `bm'
	from benchmark.rb:6:in `<main>'

It looks like raise_on_missing_parameter wasn't passed as a keyword argument in #11. After fixing that, I ran into:

st-froydnj2:message-format-rb froydnj$ RBENV_VERSION=3.1 bundle exec ruby benchmark.rb
       user     system      total        real
parse simple message  0.625948   0.003741   0.629689 (  0.629774)
format simple message/Users/froydnj/stripe/message-format-rb/lib/message_format.rb:21:in `format': undefined method `call' for nil:NilClass (NoMethodError)

      @format.call(args)
             ^^^^^
	from benchmark.rb:17:in `block (3 levels) in <main>'
	from benchmark.rb:16:in `times'
	from benchmark.rb:16:in `block (2 levels) in <main>'
	from /Users/froydnj/.rbenv/versions/3.1.4/lib/ruby/3.1.0/benchmark.rb:296:in `measure'
	from /Users/froydnj/.rbenv/versions/3.1.4/lib/ruby/3.1.0/benchmark.rb:378:in `item'
	from benchmark.rb:14:in `block in <main>'
	from /Users/froydnj/.rbenv/versions/3.1.4/lib/ruby/3.1.0/benchmark.rb:176:in `benchmark'
	from /Users/froydnj/.rbenv/versions/3.1.4/lib/ruby/3.1.0/benchmark.rb:208:in `bm'
	from benchmark.rb:6:in `<main>'

This problem also comes from #11, which inadvertently changed the return value of Interpreter#interpret. The second commit in this PR fixes that problem, after which the benchmarks run successfully.