"While amici contend that to receive proper medical care in the form of an abortion approved and performed by a physician is, by itself, a fundamental interest protected by the Constitution, it is not necessary for abortion to be declared a constitutional right to hold that its discriminatory denial violates guarantees of equal protection. The State of Texas has prohibited all abortions except for “the purpose of saving the mother’s life.” On its face, this permits treatment in the case of all women whose lives are similarly endangered, and excludes from treatment all others."