Low-provenance Expr->SyntaxTree conversion

[mlechu]

The main part of #17:

As discussed, this conversion is a bit ugly, but necessary for backwards compatibility with existing macros.

Provenance is determined by using the latest seen LineNumberNode in the Expr traversal. I don't think the line number is well-defined for all subexpressions in general, but this method should work for the obvious cases (i.e. using a linenode that immediately precedes the current expr, or its parent if none, etc.)

TODO: This will need updates whenever we start using the latest dev version of JuliaSyntax

[c42f]

Looks great. There's a few things which can be refined (such as replacing @assert with something more graceful) but those can be done later.

One thing I noticed is that we need special provenance rules for Expr(:function) - the first LineNumberNode in the body block is the location where the function was defined. So function foo() end has provenance even though there's no place in the :function Expr for the line number. This is how Base.functionloc() works.

Also, could do with some light tests that basic line number provenance is working.

[c42f]

Awesome, I think this is good to go :)