refactor rewriteJs and rewriteCoffee

[davidchambers]

I made some of these internal changes a few years ago. 🙈

Here is the new algorithm (with slight variations due to differences in the way comments are located):

$ awk '$2 ~ /rewrite(Js|Coffee)/ { print $2; look = 1 } $0 == "};" { look = 0 } look && $1 == "//" && $2 ~ /:$/ { print }' lib/doctest.js
rewriteJs
  // 1: Parse source text to extract comments
  // 2: Preserve source text between comments
  // 3: Extract prefixed comment lines
  // 4: Coalesce related input and output lines
  // 5: Convert doctests to source text
  // 6: Sort verbatim and generated source text by original offsets
  // 7: Concatenate source text
rewriteCoffee
  // 1a: Extract prefixed comment lines
  // 1b: Preserve other lines
  // 2: Coalesce related input and output lines
  // 3: Convert doctests to source text
  // 4: Sort verbatim and generated source text by original line numbers
  // 5: Concatenate source text

The change to the test suite reflects the removal of transformComments and substring and their accompanying doctests.

[Avaq]

I did a superficial review and this LGTM: You say it's just a refactor, the code looks alright, the number of lines has been halved, and the tests still pass. If I need to do a more thorough review let me know.