α‐Melanotropin Labelled at its Tyrosine2 Residue: Synthesis and Biological Activities of 3′‐Iodotyrosine2‐,3′‐125Iodotyrosine2‐,3′,5′‐Diiodotyrosine2‐, and (3′,5′‐3H2)tyrosine2‐α‐Melanotropin, and of Related Peptides