Use native Object.assign instead of xtend

[jimmywarting]

xtend can easily be replaced with Object.assign

This reduce the dependencies and possible also avoid duplicated versions where some package don't use the same version range for the xtend package

// immutable
Object.assign({}, a, b)

// mutable
Object.assign(a, b)

[twada]

@jimmywarting Thanks! I'll replace it in power-assert v2, dropping support for super-old environments.