Add helper methods for creating and finding method and field nodes

[Jonathing]

This PR aims to provide useful methods for creating and finding method nodes and field nodes.

• Added sister methods to getMethodNode() with additional parameters.
• Added getFieldNode(), a spiritual sister to getMethodNode() but for field nodes instead.
• Added findMethodNode(), a method which searches the given class node for a specific method.
• Added findFieldNode(), a method which searches theg given class node for a specific field.

[Jonathing]

Closing this as all these changes are rather small and are included in the bigger PR: #54.