Make bundle filename consistent

[ehmicky]

Fixes #50.

At the moment, the filename of a bundle is always different each time bundling is performed, even if the Edge handlers source files have not changed. This might be a problem:

• This creates unnecessary additional bundles in .netlify/edge-handlers/ even when nothing has changed
• This prevents using the bundle filename as an indication of whether any Edge handlers contents has changed, which could be interesting as a performance/caching optimization

The reason the bundle filename always changes is because each Edge handler is exported using a unique ID. That ID is part of the final bundle and is randomly generated.

This PR solves this problem by generating this unique ID by performing a SHA1 cheksum on the Edge handler filename instead.
If any Edge handler source file's contents changes, the bundle filename will still change since it is based not only on each generated ID, but also on a concatenation of all Edge handlers source files.

One side-effect this PR is that id in netlifyRegistry.set("handlerFilename", "id") will remain the same across builds if the handler filename has not changed. However, it is guaranteed to be unique within a specific build, since two Edge handlers cannot have the same filename (full file paths can also be used in the future). Please let me know if this is a problem with how netlifyRegistry is implemented. I am guessing not but I want to confirm.

[calavera]

👌

[mraerino]

oh, this is very valuable. thank you!