[Grammar] Flow Control Statements

[AtomCrafty]

Implements proposals 4 and 5 from #37.

Structure Changes

• Allow switch statements without any sections
• Split JumpStatement into ContinueStatement, BreakStatement and ReturnStatement

Name Changes

Old Class	Old Field	New Class	New Field
IfStatement	cond	IfStatement	condition
IfStatement	thenStmt	IfStatement	thenBody
IfStatement	elseStmt	IfStatement	elseBody
SwitchStatement	cond	SwitchStatement	condition
SwitchStatement	items	SwitchStatement	sections
LabeledStatement	items	SwitchSection	body
LabeledStatement	constExpr	CaseSection	condition
JumpStatement	expr	ReturnStatement	value