Relicense shootout-pidigits.rs

Everyone agreed.

Related to #14248, close #14718
Relicense shootout-binarytrees.rs

Everyone agreed.

Related to #14248, close #14720
auto merge of #14852 : TeXitoi/rust/relicense-shootout-pidigits, r=brson

Everyone agreed.

Related to #14248, close #14718

@brson OK?
auto merge of #14855 : TeXitoi/rust/relicense-shootout-binarytrees, r=brson

Everyone agreed.

Related to #14248, close #14720

@brson OK?