Features
* New shim version meta-data allows shims to not depend on a particular plugin
nor on its relative executable path (#431)
Upgrading requires shim re-generation and should happen automatically
by `asdf-exec`:
`rm -rf ~/.asdf/shims/` followed by `asdf reshim`
* Added lots of tests for shim execution.
We now make sure that shim execution obeys plugins hooks like
`list-bin-paths` and `exec-path`.
* Shim exec is now performed by a new `bin/private/asdf-tool-exec` that might
be faster for most common use case: (versions on local .tool-versions file)
but fallbacks to slower `get_preset_version_for` which takes legacy formats
into account.
* Shim exec recommends which plugins or versions to set when command is
not found.
Fixed Bugs
* Allow many plugins to provide shims with same executable name (#431)
