compiler: bugfix: incorrect scoping in macro param binding.

The destructuring binder binds all of the variables in the
template to nil values and then assigns to them as it walks
the object that is being destructured. Unfortunately, this
results in incorrect treatment of init-forms, which are
evaluated in the wrong scope. They are actually evaluated in
completely the wrong scope due to the use of up:env, but the
problem can't be fixed by removing up:env.

The approach here is to generate a big let* construct that
binds the variables in sequence, rather than assigning to
them.

* share/txr/stdlib/compiler.tl (expand-bind-mac-params):
The basic structure of the code remains the same, but the
details are rewritten. Instead of emitting a body of forms, we
emit let* bindings. Because some of the logic requires
imperative statements, like stepping pointers through the
destructured object, these are mixed into the variable
initializations via progn. The local functions emit-stmt and
emit-var provide the interface for doing this.  There is a bit
of trickery in the situation that an optional parameter also
has the presence-indicator variable. We must bind the
parameter in an environment in which that presence-indicator
variable is not yet visible. This is achieved by binding the
variable to a nil value, and then binding the presence
indicator to an expression which sets the variable's value as
a side effect, and yields a Boolean value that initializes the
indicator.

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