Rewrite the parser for the configuration structure to allow laout or braces

Joint work with Thomas Schilling.
The sections and indeed fields (and if/else) can now use either explicit
brace {} style layout or indentation eg:
> library
>   exposed-modules: Blah
or
> library {
>   exposed-modules:
> }
layout style can be nested within explict braces style and vica versa.
Also add some more checks and relax the tab checks.
Unrecognised sections, like unrecognised fields, are not fatal errors,
so we could add sections in future without breaking old cabal.