In type constraints, object attributes may be marked as optional in
order to allow them to be omitted from input values. Doing so results in
filling the attribute value with a typed `null`.
This commit adds a new type `typeexpr.Defaults` which mirrors the
structure of a type constraint, storing default values for optional
attributes. This will allow specification of non-`null` default values
for attributes.
The `Defaults` type is a tree structure, each node containing a sub-tree
type, a map of children, and for object nodes, a map of defaults. The
keys in the children map depend on the type of the node:
- Object nodes have children for each attribute;
- Tuple nodes have children for each index, with indices converted to
string values;
- Collection nodes have a single child at the empty string key.
When traversing this tree we must take this structure into account, with
special cases for map input values which may later be converted to
objects.
The traversal defined in this commit uses a pre-order transformer in
order to pre-populate descendent nodes before their defaults are
applied. This allows type nested type default values to be specified
more compactly.
