Support PHP 7.4 arrow functions (fn($x) => expr)

[TysonAndre]

These have similar properties to closures, except:

• There is a single expression instead of a function body
• There is no use clause, it's implicit

More details can be found in https://wiki.php.net/rfc/arrow_functions_v2

Arrow functions have lowest precedence. This means that the expression to the right of => will be consumed as far as possible:

Examples:

fn(array $x) => $x;
fn(): int => $x;
fn($x = 42) => yield $x;
fn(&$x) => $x;
fn&($x) => $x;
fn($x, ...$rest) => $rest;
static fn() => 1;

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Ideas on how this would be represented:

Create a new ArrowFunctionCreationExpression with staticModifier, FunctionHeader, FunctionReturnType, arrowToken, and resultExpression: Token|Node (instead of FunctionBody). There would be no FunctionUseClause.

It would have these properties in this order (when the ast is valid):

• staticModifier: Token|null
• functionKeyword: Token (to reuse FunctionName, but fn instead of function)
• byRefToken: Token|null
• openParen: Token
• parameters: DelimitedList\ParameterDeclarationList
• closeParen: Token
• colonToken: Token|null
• questionToken: Token|null
• returnType: Token|qualifiedName
• arrowToken: Token (=>)
• resultExpression: Token|Node