FIXME: this currently fails the test suite

This change has no effect at present because AliasStmt::validate is not
implemented, but for consistency and to be future proof this should have been in
place.

We provide an API supporting range-based for loops and legacy iteration over an
AST or a subtree rooted at a particular node. The purpose of this is to abstract
patterns like indexing and validation into a common implementation. This needs a
const counterpart, but then it should be ready to near-entirely replace the
functionality of indexing, validation, and friends. This should then be a
suitable base to build Node::all, Node::any, Node::none, etc on top of.

Github: related to #152 "traversal abort"

I think these were leftover from when the visitation function was overloaded.

We have an upcoming need to rewrite parts of the AST (overwrite one AST node
with another). This would be easier done if we could return a new Node from a
visitation function. To achieve this we need a tweakable return type as part of
the visitor base classes.

Note that we cannot make the intermediate children of BaseTraversal and
ConstBaseTraversal (e.g. Traversal) also template classes, because there is no
straightforward way to construct a value to return when the template type is
instantiated as non-void. We could play some tricks with template specialisation
but it seemed easier to just leave them as concrete classes.

This lets you more easily run only the tests for rumur or only the tests for
rumur-ast-dump. This will become useful as we move to having another binary, a
preprocessor, in the mix soon.

Filename's like `a quote: " <-` would have caused invalid XML to be generated
previously.

We now *always* have a filename attribute on the unit tag.

We now have full model content when piping in a model from stdin as well as when
the model arrives via a file.

It seems the stream that is used for parsing is unusable afterwards. Flex or our
own code has caused failbit to be set. Instead of trying to undo this, simply
use another stream. We'll make use of this for replay in an upcoming commit.

This is part of work towards including the input model content with the AST
produced by rumur-ast-dump, regardless of whether the model is provided in a
file or on stdin.

This was intended to be a parent of master already, but I seem to have somehow
fouled this up in a previous packaging cycle.

Github: closes #168 "add xmllint and an SMT solver to FreeBSD testing"

This is required to run the ast-dump tests.

Github: related to #168 "add xmllint and an SMT solver to FreeBSD testing"

When building in debug mode, the user could trigger an assertion failure by
setting a value type that was too small to contain values in the model. Note
that this would *only* occur when the user specifically overrode --value-type
with a narrower type than the one that would have been selected automatically.

Github: closes #166 "users can force verifier assertion failure"

We're about to introduce a new error case in these functions.

Github: related to #166 "users can force verifier assertion failure"

If value_t is chosen to be an unsigned type, comparisons like `v < 0` cause
warnings with -Wtype-limits. We also need to work around a -Wsign-compare bug in
this change.

Github: related to #163 "reverse for ranges don't work"

We should now have full support for incrementing by negative amounts within a
quantifier.

Github: closes #163 "reverse for ranges don't work"

The lower bound may be above the upper bound if the user has described a down
counting loop. These aren't properly supported yet, but it is an aspiration.

Github: related to #163 "reverse for ranges don't work"

This is a bit of a happy accident here. I thought we were explicitly requiring
the step of a for loop to be a generation-time constant, as CMurphi does.
However, it turns out we were actually constant folding the step during
validation, and as a side effect throwing an error during constant folding when
we encountering something non-constant.

This commit fixes validation to only attempt constant folding of the step when
it is actually foldable. We still need some generated validation checks for
loops that have dynamic bounds and/or steps.