Improve keyword arguments test and key.

[okamsn]

• Document the argument order of test as the first argument being the value
  from the sequence and the second argument being the tested item. This is the
  same order as used by seq-contains-p and the opposite of cl-member.

  • Add loopy--member-p, used for tests for adjoin, union, and nunion.
    This function can be optimized during byte compilation via
    loopy--member-p-comp to become member, memq, or memql when possible,
    as done with cl-member.

• Simplify loopy--plist-bind into a wrapper around cl-destructuring-bind.

• Add loopy--instr-let2*, with works like macroexp-let2* except that it will
  also append variable-instructions to the result of the body as needed. This
  will allow us to stop manually checking in each command whether we need to
  create a variable to hold a value. Now, we create the variable is
  macroexp-const-p believes that the value would not be constant.

• Do not remove key. The use of a separate argument allows us to optimize the
  transform function by only calling it once on the tested item
  during the execution of each command. It is decided that re-creating
  this behavior with set and an ignored argument in the test function
  is too awkward.

See also issues #176, #170, and this PR.