Closes #34

This moves dagrt.codegen.data to dagrt.data, and adds more
documentation to the module.

SymbolKindFinder is changed to have a slightly more generic interface,
accepting iterables instead of ASTs.

This also adds a helper function for kind inference of DAGs.

This depends on !37

Use pytools.natsorted

See merge request !38

This improves the documentation of dagrt.function_registry. It also
exposes the documentation for the built-in functions.

What's not yet incorporated:

 * changes to isnan (#30)
 * changes to the type inference (#34)
 * any interface changes due to #24

Improve code stringification

See merge request !35

This makes code stringification deterministic, makes the initial phase
clear in the stringified code, and renames STAGE in the output to
PHASE.

Fix bitrot in example; add Examples CI job

See merge request !34

CodeBuilder documentation fixes

See merge request !33

codegen.expressions: Remove deprecated uses of constant_mapper

See merge request !32

Remove RestartStep from the Dagrt language; some DAGCode changes

See merge request !30