vec: Write the .extend() specialization in cleaner style
vec: Write the .extend() specialization in cleaner style

As far as possible, use regular `default fn` specialization in favour of
ad-hoc conditionals.

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