Simplify building the graphql-anywhere/lib/async entry point.

[benjamn]

Should fix apollographql/apollo-link-rest#194.

In graphql-anywhere@4.2.0, the graphql-anywhere/lib/async.js module accidentally disappeared, with graphql-anywhere/lib/async.cjs.js in its place.

Whatever you may think about the wisdom of sub-module imports, graphql-anywhere/lib/async was expected to exist by other packages like apollo-link-state, so we should fix it.

This commit removes the special Rollup step for async.cjs.js and renames src/graphql-async.ts to just src/async.ts so that tsc will output graphql-anywhere/lib/async.js as CommonJS, as desired.