Parse more stabby procs without parentheses

[FnControlOption]

Parse parentheses-less stabby proc with first argument as:

• keyword splat,
• block argument,
• optional argument,
• optional keyword argument, or
• mandatory keyword argument:

-> **foo { 1 }
-> &foo { 1 }
-> foo = bar { 1 }
-> foo: bar { 1 }
-> foo: { 1 }

Also parse parentheses-less def with block arg:

def foo &a; end

[seven1m]

Nice