feat: unbuffered channel

[shenyih0ng]

Description

This PR implements unbuffered channel support for the Go ECE. For an unbuffered channel, the sender blocks until the receiver has received the value (https://go.dev/doc/effective_go#channels).

Implementation Details

1. An unbuffered channel in the Go ECE is represented using 2 words (2*8 = 16 bytes) including the tagged pointer

   • We store the ids of the sender and receiver goroutines in the tagged pointer, each occupying 2 bytes, hence we are assuming an upper bound of 2^16 goroutines in total
   • The one word (8 bytes) that follows the tagged pointer will be used as a slot for senders to send to and receivers to receive from
   • We also use 1 byte in the tagged pointer to track if there is a sync process happening
     • More details of what the sync process is can be found in the UnbufferedChannel implementation (i've left a bunch of comments to explain the behaviour)

2. Unlike the actual implementation of channels in go (https://docs.google.com/document/d/1yIAYmbvL3JxOKOjuCyon7JhW4cSv1wy5hC0ApeGMV9s/pub) that relies on mutexes, I opted to model channels as its own independent construct. This means that the unbuffered channel itself is responsible for signalling/blocking goroutines.