I was hoping to add nested datatypes into the language, but it turns out
doing so is quite tricky and not all that useful with Roc's current
compilation model. Basically every implementation strategy I could think
of ended up requiring a uniform representation for the data layout
(or some ugly workaround). Furhermore it increased the complexity of the
checker/mono IR generator a little bit - basically, we must always pass
around the alias definitions of nested datatypes and instantiate them
at usage sites, rather than being able to unroll aliases as we currently
do during canonicalization.
So, especially because we don't support polymorphic recursion anyway, I
think it may be better to simply disallow any kind of nested datatypes
in the language. In any case, Stephanie Weirich [seems to think nested
datatypes are not needed](https://www.cis.upenn.edu/~plclub/blog/2020-12-04-nested-datatypes/).
Closes#2293
This may be false e.g. in the presence of a multiline string. Since the
parser is in a more stable state than when this was introduced, I think
this is OK to remove.
Closes#2398
* Remove the `pos` field, which was always being assigned Position::default()
* Remove one use of this `pos`, by removing the never-used SyntaxError::ConditionFailed variant
* Adjust the other use to do what was probably intended - which is to say, pointing to the beginning of the def with the error
* Rename to FileError, reuse `SourceError` as an inner field, to avoid duplicating the `bytes`
