Move secp256k1_fe_one to field.h

This makes secp256k1_fe_one part of field.h's interface, and allows other modules to appropriately access the constant.

src/field.h

@@ -32,6 +32,8 @@
 #error "Please select wide multiplication implementation"
 #endif
 
+static const secp256k1_fe secp256k1_fe_one = SECP256K1_FE_CONST(0, 0, 0, 0, 0, 0, 0, 1);
+
 /** Normalize a field element. This brings the field element to a canonical representation, reduces
  *  its magnitude to 1, and reduces it modulo field size `p`.
  */