Improve ResolvableType.hashCode() for better performance

Prior to this commit, when there was a lot of entries in the
ResolvableType.cache HashMap, getting a simple value could
take a lot of time due to a lot of calls to ResolvableType.equals().
ResolvableType.equals() used this.type, getSource(),
this.variableResolver.getSource() and this.componentType, but
ResolvableType.hashCode() used only this.type.

With this commit, ResolvableType.hashCode() now uses the same
fields than ResolvableType.equals().

Performance on the spring-resolvabletype-benchmark project:
 - 8000 us before this commit
 - 120 us with this commit

Issue: SPR-12122