Encoding.default_external does not exist in Opal
Move lock_in_subs method to Substituters module
Merge pull request #719 from Mogztter/opal-absref

Move lock_in_subs to Substituters module
Merge pull request #711 from Mogztter/opal-encoding

Encoding.default_external does not exist in Opal
changes related to #679

- remove use of mutable string operations: chop!, lstrip! and rstrip!
- calculate time elapsed using floats
- use $~ instead of numbered variables in regexp substitutions
- match endline in line break regexp