Parsing here-docs
basic hereDocs!
Support a proxy for tab characters in doctests
here doc tests
content expansion support
here-doc expansion
expansion for ENV and PS1 variables
honor live flag during parameter expansion
backtick command substitution

My reading of the spec implies that backticks must be parsed by slewing
the text to the matching backtick, and only afterwards parsing. In this
implementation, the parsing is delayed until expansion is performed,
which may or may not be the right thing.

missing file
Command substitution for the $( ) form

My reading of the spec is that the contents in this form must actually
be parsed. This is probably more correct w.r.t. here-documents.
tests for command substitution and a comment
Clean up command substitution code a bit

The cycle of execution -> expansion -> execution (for command substitution) can be broken by just the Execute.hs-boot file. So Remove Script.hs-boot.
Enable the command substitution test in the here-document test