Fixed typos and format inconsistencies in headers of various RFCs.

In particular:

* The RFC associated with rust-lang#127 should have had a link to rust-lang#19 as well
  (and has been assigned RFC rust-lang#19); it also was revised to match the
  markdown href style of other RFCs.

* RFC rust-lang#34 needed its header entries filled in,

* RFC rust-lang#123 had a typo in its header, and

* RC rust-lang#155 was revised to match the markdown href style of other RFCs.

text/0019-opt-in-builtin-traits.md

@@ -1,6 +1,6 @@
 - Start Date: 2014-09-18
-- RFC PR #: #127
-- Rust Issue #: #13231
+- RFC PR #: [rust-lang/rfcs#19](https://github.com/rust-lang/rfcs/pull/19), [rust-lang/rfcs#127](https://github.com/rust-lang/rfcs/pull/127)
+- Rust Issue #: [rust-lang/rust#13231](https://github.com/rust-lang/rust/issues/13231)
 
 # Summary
 

text/0034-bounded-type-parameters.md

@@ -1,6 +1,6 @@
 - Start Date: 2014-04-05
-- RFC PR:
-- Rust Issue:
+- RFC PR: [rust-lang/rfcs#34](https://github.com/rust-lang/rfcs/pull/34)
+- Rust Issue: [rust-lang/rust#15759](https://github.com/rust-lang/rust/issues/15759)
 
 # Summary
 

text/0123-share-to-threadsafe.md

@@ -1,5 +1,5 @@
 - Start Date: 2014-06-15
-- RFC PR #: [rust-lang/rfcs#123](https://github.com/rust-lang/rfcs/pull/123j)
+- RFC PR #: [rust-lang/rfcs#123](https://github.com/rust-lang/rfcs/pull/123)
 - Rust Issue #: [rust-lang/rust#16281](https://github.com/rust-lang/rust/issues/16281)
 
 # Summary

text/0155-anonymous-impl-only-in-same-module.md

@@ -1,6 +1,6 @@
 - Start Date: 2014-07-04
-- RFC PR #: 155
-- Rust Issue #: 17059
+- RFC PR #: [rust-lang/rfcs#155](https://github.com/rust-lang/rfcs/pull/155)
+- Rust Issue #: [rust-lang/rust#17059](https://github.com/rust-lang/rust/issues/17059)
 
 # Summary