directed 0.2.4

Evaluate programs based on Directed Acyclic Graphs
Documentation
# 0.2.4
- Various bugs fixed.

# 0.2.0
- Massive rewrite.

# 0.1.17

- Inputs to top-level nodes can now be injected.

# 0.1.16

- Graph outputs can now be accessed, but still a bit too primitive.

# 0.1.15

- True async stages now implemented.

# 0.1.14

- Made the `graph` macro more clean.
- Now can make connections between nodes with no associated data.

# 0.1.13

- Fixes for some broken parts of `cache_all`

# 0.1.9

- Very basic implmenetation of `cache_all`, still lacking comprehensive tests

# 0.1.7

- First implementation of the `tokio` optional feature. Async works!
- Fixed a few debilitating compile-time problems