Relax version requirements to allow building on modern installations
Make tests compile again

To do so, add needed imports and update tests to use the new equality
definition. The test now fail when comparing terms with division by
zero - the division is not residualized.
Normalisation fails on division by zero: add testcase
Remove dead code to not maintain it

Otherwise, this code would stop compiling with next commit.
Fix normalization: residualize division by zero
Merge pull request #6 from Blaisorblade/master

Make tests compile again, residualize division by zero
Refactor stmt to ease bugfix: reduce duplication

Use the optionMaybe combinator. This reduces code duplication and
left-factorizes the grammar somewhat, potentially improving error
messages. Consider parsing:

foo : * = Pi T . T;

This statement is incorrect, because the grammar requires Pi terms to
have a type annotation, as in `Pi T : * . T`. However, due to
backtracking, we get the wrong error here.

With this fix, when parsing:

foo : * = Pi T . T;

we will no more not try parsing this as `ident args '=' expr` and get a complaint about ": *" (see issue #9).

However, this grammar still requires backtracking, since expressions
can well start with identifiers. So, on the example above, we'll
backtrack and try to parse it as `expr`, and we'll still complain
about the ':'.

Using left-factoring for that seems like a nightmare, subsequent
commits will have a different solution.
Refactor stmt to allow bugfix: push down semicolon

See next commit for a rationale.
Fix #9: Try expressions first, backtrack to definitions

To get the right backtracking behavior (in our case), we need to try
parsing a statement as a definition only after all other alternatives,
that is, after checking if it's a valid expression. This way, if
parsing as a definition fails, we don't backtrack to anything
else.

This might make error messages worse for expressions, but unless
a better fix can be found that's a reasonable tradeoff, because
definitions are often more complex than expressions. As a workaround,
turning `expr;` into `freshName = expr; freshName;` will improve the
error message on `expr`.

However, this would not work without the previous commit: the
beginning of every definition is a syntactically valid expression, so
`try expr` never fails. However, `expr` will not parse the whole
definition; to turn this into a failure, the simplest approach is to
push the parser for the semicolon inside the alternative productions
for statements and inside the try, as done in the previous commit.

Conceptually, pushing a semicolon into the branches should not make a
difference, but that's only valid for CFGs. Even LL(inf) parsers would
have the same problem - (a ';' | b ';') and (a | b) ';' have different
behaviors.

The need for such a transformation points to a modularity problem in
Parsec: you can't give them a compositional semantics mapping them to
CFGs. But the only way to achieve that in general would be using
generalized parsing.
Issue #10: Accept -q/--quiet option - don't use it yet
Issue #10: Implement --quiet, v1: avoid most output

Messages like "process file X.pts" are still output. See next commit.
Fix #10: Make pts completely quiet when requested

This requires moving the "process file X" into another function (which
has access to the options through a reader monad) and writing a
reusable outputLine function - a variant of putStrLn which is disabled
by --quiet, and runs in this reader monad.
Add missing type signature
Merge pull request #12 from Blaisorblade/issue/10

Fix #10: Make pts completely quiet when requested by new --quiet option
Merge pull request #11 from Blaisorblade/issue/9

Fix #9: try expressions first, backtrack to definitions
Add Paolo as author.
Reorganized file structure
Relaxed build dependencies
Put a README.md with installation instructions under track
Set the upper bound for some dependencies