Implementation of single and bulk deletion from KV

[gabbifish]

This PR closes #344 and #347. It implements single and bulk deletion of values from Workers KV.

This PR may potentially still be WIP because we still need to decide how exactly we want the workers subcommands to look like, especially between {namespace, individual, bulk} deletion; we can do this in this PR or on the feat-kv-commands branch.

So far, the commands are used as the following:
Individual delete:

$ wrangler kv delete-key 06779da6940b431db6e566b4846d64db matt
 Deleting key "matt"
 Success

For bulk delete (on a json blob):

$ wrangler kv delete-bulk 06779da6940b431db6e566b4846d64db test-delete.json 
 Success

For bulk delete (on a directory):

$ wrangler kv delete-bulk 06779da6940b431db6e566b4846d64db abc
 Deleting a.txt...
 Deleting b%2Fc.txt...
 Success

This PR will fail to build until the necessary API changes are also merged into cloudflare-rs (cloudflare/cloudflare-rs#29), because it is dependent on these changes.