VaList::copy should not require a mutable ref
VaList::copy should not require a mutable ref

VaList::copy does not need to take a mutable reference. The va_copy
intrinsic takes a immutable reference.

graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust