Implement String#undump in C extension library

[tadd]

Add String#undump_roughly, a C extension implementation of String#undump.

Now undump with Ruby trunk is over 9 times faster than eval 🎉

$ cat bm.rb 
require 'string_undump'

def measure1
  t = Time.now
  N.times{yield}
  Time.now - t
end

def measure(&block)
  3.times.map{measure1(&block)}.min
end

N = 100_000
s = 'あいうえお'.dump

print "eval: "
e = measure{eval(s)}
puts e

print "undump: "
u = measure{s.undump}
puts u

puts "undump is #{e/u} times faster!"
$ ruby-trunk bm.rb 
eval: 0.502793878
undump: 0.051393841
undump is 9.783154327772465 times faster!

[tadd]

Yes, the name of this library is renamed from string-undump (hyphen) to string_undump (underscore) due to naming convention of C extension.

[tadd]

Ruby implementation remains as String#undump_badly.