Deprecate and replace 're' with 'nre'

[oprypin]

Nim's regular expression library (re) has many problems.

• Broken behavior
  • Numerous problems with 0-length matches in 're'
• Not supported:
  • Unicode
    • This not only means that character classes like \w are limited (\w is just A to Z), but also that bytes of one character can be separated.
  • Replacing with a function
  • Splitting with captures
  • Countless PCRE flags
• Limited to 20 captures
• Unintuitive API
  • Many functions that do almost the same thing but differently
  • Requires passing a pre-populated var argument
  • Two flags are on by default, so if you want to add one more you must specify three flags manually.

Many people report problems chaotically in different places, some do pull requests, sometimes changing from a broken behavior to a wrong behavior, sometimes adding more procedures, making the API more confusing. This has gone on for too long.

There is a new library, nre. It solves pretty much all of the problems mentioned above. It is very polished and thoroughly tested for all corner cases. It has a consistent, concise but richer API.

I suggest to include nre in the standard library.

Steps towards implementing this:

• Implement nre
• Change nre to use inclusive upper bounds
• Encourage people to use nre in re's documentation
• Remake nre's documentation to use Nim's docgen / reST
• Update standard library's PCRE wrapper
• Use standard library's PCRE in nre
• Reimplement re based on nre

After 0.11.2 release:

• Drop nre's support for locally compiled PCRE
   Move optional_t (a dependency) into standard library
• Implement an options library in stdlib
• Change nre to use options
• Move nre into standard library
   Replace re with the wrapper around nre, thus getting rid of its bugs and maintenance burden
• Deprecate re

[dom96]

Relevant discussion on IRC from about 6pm onwards: http://irclogs.nim-lang.org/10-04-2015.html

[Araq]

Well I need to review these patches and test them still, but apart from that it's all fine with me.

[flaviut]

Blocked on #2515.