Add simple statement parsing.
Add declaration and labeled statement parsing.
Move statement_test.go to the shorter stmt_test.go.
Test empty statement parsing.
Add parsing of goto, continue, and break statements.
Rename *Statement nodes to *Stmt.

This is what the spec calls them, and it's shorter.
Allow call expressions with empty parameter lists.
Add parsing of return, defer, go, and fallthrough statements.
Add block parsing.
Add a test for FallthroughStmt parsing.