Provide types or helpers for common deref operations

[frangio]

Let deref: ASTDereferencer, we could have additional helpers:

• deref.functionCall(node: FunctionCall): FunctionDefinition | VariableDeclaration | undefined
• deref.scope(node: FunctionDefinition | StructDefinition | ...): ContractDefinition | SourceUnit
• deref.scope(node: VariableDeclaration): Block | ContractDefinition | SourceUnit
• (...others)

Alternatively, we can provide arrays of node types for each of these use cases that could be passed in to deref function.

[ericglau]

From testing, I found the following although I'm not sure if these are exhaustive:

• A FunctionCall's expression.referencedDeclaration can reference any of the following types: 'FunctionDefinition', 'EventDefinition', 'ContractDefinition', 'StructDefinition'
• A ContractDefinition's scope can reference any of the following types: 'ContractDefinition', 'SourceUnit'

[ericglau]

FunctionCall's expression.referencedDeclaration can also reference 'VariableDeclaration', 'ErrorDefinition'