generate-json-rpc-spec: move into own crate and fix when run at repo …

[bmwill]

…root

This PR does 4 things:

1. Move generate-json-rpc-spec functionality into its own crate
2. Makes generate-json-rpc-spec a binary instead of an example
3. Changes the integration test which ensures the checked-in format has been updated to use the binary pre-compiled by cargo before the test is invoked. This eliminates the odd extra compile that occurred when running this test
4. Fixes the binary to not be depended on the directory it's run from in order to function properly.

[lxfind]

Cool. Line 130 in main.rs seems to use a hardcoded path as well