deprecation warning P0, P1, P2

[Leopold Toetsch]

Here's again a summary of upcoming changes. I'll commit for now just the
new "returncc" opcode. Other changes that start breaking things will
follow in a few days.

Summary:

1) PIR code using PIRs HLL call syntax doesn't need changes.

foo() # call sub foo
a = bar(x) # call bar with argument, get return value
(a,b,c) = baz(x,y)

.return() # return from a sub - maybe omitted if last
# statement before .end
.return(foo, bar) # return these values

.yield(foo) # yield value(s) from a coroutine

self."method"(x, y) # method call on object self
$P0 = other_object # and on another one
(r, s) = $P0."other_meth"(a)

2) PASM code using pdd03 calling conventions

All explicit usage of P0, P1 related to subroutines as well as P2 in OO
code is *not* safe and need changes.

2a) return from a sub

instead of "invoke P1" use the new opcode "returncc"

2b) accessing the object "self" in methods:

.include "interpinfo.pasm"
interpinfo P2, .INTERPINFO_CURRENT_OBJECT
# get object into given register

2c) access the current subroutine object

.include "interpinfo.pasm"
interpinfo P0, .INTERPINFO_CURRENT_SUB

2d) create a real continuation from the current continuation

.include "interpinfo.pasm"
interpinfo P1, .INTERPINFO_CURRENT_CONT

This continuation is already a full Continuation PMC usable for call/cc.

Reference:
docs/pdds/pdd03_calling_conventions.pod

I'll try to make these changes seamlessly, but I can't guarantee that
there will be no intermittend failures in tests or other code, until the
changes are done.

leo