[Relay] Toward a working VGG Net (continued).

[cgyurgyik]

VGG Net

• Change go-done interface to invokeables. This requires Dahlia #340. (Use Invoke in Relay. #379)
• Re-add the rest of the supported Relay functions. (Relay call node definitions. #396)
• Support constants ([Relay] Add constant support. #406)
• Support std_exp round 2; finish nn.softmax implementation. This should be the last piece of lowering the entire VGG Net to FuTIL. (Relay call node definitions. #396, Fixed point std_exp #404, Softmax #490)

Code Improvements

• Fix the test suite. Tests for individual Relay function calls (e.g. nn.relu) should be part of the integration test suite. However, a few unit tests should be added to ensure that any changes to Calyx affecting the Relay frontend will cause notice. (Relay call node definitions. #396)
• Fix FuTIL AST. Most of the work here should be done in Shared Python library for generating Calyx programs #368.
• Clean up frontends/relay/example.py. ([Relay] Add constant support. #406)
• Update installation. Currently missing antlr-4-python3-runtime pip install. Move to main Calyx documentation. ([Relay] docs #408)

[cgyurgyik]

Status update: Blocked on a working softmax or exp operator, as well as synthesizable {mult, div, mod}_pipes for fixed point.