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.
Add if-statement parsing.
Parse for-loop statements.
Move precedence and unary variables nearer to their uses.

They are about expressions, which are now toward the bottom of
grammar.go, not the top.
Cleanup range loop parsing.

This change factors out some code in the ugly parseSimpleStmt
function. The function still does too much, but I don't see a better
solution at the moment.
Rename the simple statement options.

Also, there is no need for them to be bitflags.
Optionally allow type switch guards to be returned by the expression parser.
Add switch statement parsing.
Set the endLoc field on switch statements.
Rename parseExpressionOpts.

It was an ugly name. Now, parseExpression is called parseExpr and
parseExpressionOpts is called parseExpression. This is nice, because
the shorter call has a briefer name, and the more verbose call has the
full, long name.
Add select statement parsing.