Rename this repo and the published module to `graph-as`

[schmidsi]

For an aspiring subgraph developer/consumer it is not obvious what graph-ts is. Just by looking at the name, people are assuming it is some helper functions to query The Graph. Like here: https://github.com/graphprotocol/graph-ts/issues/239

Therefore, I suggest to rename this repo and the published module to graph-as or even something more verbose like: subgraph-as, subgraph-assemblyscript-bindings, ...