perf build class, better show_source

[ermolaev]

class B
end

Benchmark.ips do |x|
  x.report("class_eval")         { B.class_eval("def a? = 1") }
  x.report("define_method str")  { B.define_method("#{:a}?") { 1 } }
  x.report("define_method sym")  { B.define_method(:"#{:a}?") { 1 } }
  x.compare!
end;

# Comparison:
#    define_method sym:  2324533.3 i/s
#    define_method str:  2286626.9 i/s - same-ish: difference falls within error
#           class_eval:   257038.4 i/s - 9.04x  slower