Add HashWithStructAccess#has? and HashWithStructAccess#lookup

[Michael B. Klein]

h.has?('foo.bar.baz') # true if the full path to the key exists, false if it doesn't
h.lookup('foo.bar.baz') # returns the result if it exists, nil if it doesn't

[Michael B. Klein]

Implementing #lookup as #lookup! to avoid polluting the namespace

[Michael B. Klein]

Fixed in commit 4020c35