There were 400 errors to fix and most of the mwere fixed by adding explicit type
assertions at every error site. It would be a good idea to go back and
clean some of these up later.

There are a few places that I changed the logic or the types, which I've
tried to point out in the pull request comments.

# Conflicts:
#	src/Chunk.ts
#	src/ExternalModule.ts
#	src/Graph.ts
#	src/Module.ts
#	src/finalisers/amd.ts
#	src/finalisers/cjs.ts
#	src/finalisers/iife.ts
#	src/finalisers/umd.ts
#	src/rollup/index.ts
#	src/utils/assetHooks.ts
#	src/utils/chunkColouring.ts
#	src/utils/defaultPlugin.ts
#	src/utils/pluginDriver.ts
#	tsconfig.json