Add support for explicit, nested scopes

[bbannier]

It currently seems impossible to introduce nested scopes.

module foo;

function f() {
    {
        local x = 1;
    }
}

$ spicyc -dj foo.spicy
[error] foo.spicy:4:5: syntax error, unexpected '{'
[error] spicyc: parse error

We should consider introducing support for this as it can e.g., help avoid name pollution by limiting variable scope.

The current workaround is to use e.g., an if with a block.