Initial attack at basic 'class' feature

Adds a new experimental warning, feature, keywords and enough parsing to
implement basic classes with an empty `new` constructor method.

Inject a $self lexical into method bodies; populate it with the object instance, suitably shifted
Creates a new OP_METHSTART opcode to perform method setup
Define an aux flag to remark which stashes are classes

Basic implementation of fields.

Basic anonymous methods.

MANIFEST

@@ -18,6 +18,7 @@ caretx.c		C file to create $^X
 cflags.SH		A script that emits C compilation flags per file
 Changes			Describe how to peruse changes between releases
 charclass_invlists.h	Compiled-in inversion lists
+class.c			Internals of the `use feature 'class'` object system
 CODE_OF_CONDUCT.md	Information on where to find the Standards of Conduct
 config_h.SH		Produces config.h
 configpm		Produces lib/Config.pm
@@ -5638,6 +5639,12 @@ t/bigmem/regexp.t		Test regular expressions with large strings
 t/bigmem/subst.t		Test s/// with large strings
 t/bigmem/vec.t			Check vec() handles large offsets
 t/charset_tools.pl		To aid in portable testing across platforms with different character sets
+t/class/class.t			See if class declarations work
+t/class/construct.t		See if class constructors work
+t/class/destruct.t		See if class destruction works
+t/class/field.t			See if class field declarations work
+t/class/method.t		See if class method declarations work
+t/class/phasers.t		See if class phaser blocks work
 t/cmd/elsif.t			See if else-if works
 t/cmd/for.t			See if for loops work
 t/cmd/mod.t			See if statement modifiers work
@@ -5721,6 +5728,7 @@ t/lib/common.pl			Helper for lib/{warnings,feature}.t
 t/lib/commonsense.t		See if configuration meets basic needs
 t/lib/Count.pm			Helper for t/op/method.t
 t/lib/croak.t			Test calls to Perl_croak() in the C source.
+t/lib/croak/class		Test croak calls from class.c
 t/lib/croak/gv			Test croak calls from gv.c
 t/lib/croak/mg			Test croak calls from mg.c
 t/lib/croak/op			Test croak calls from op.c
@@ -5787,6 +5795,7 @@ t/lib/warnings/9enabled		Tests warnings
 t/lib/warnings/9uninit		Tests "Use of uninitialized" warnings
 t/lib/warnings/av		Tests for av.c for warnings.t
 t/lib/warnings/builtin		Tests for builtin.c for warnings.t
+t/lib/warnings/class		Tests for class.c for warnings.t
 t/lib/warnings/doio		Tests for doio.c for warnings.t
 t/lib/warnings/gv		Tests for gv.c for warnings.t
 t/lib/warnings/hv		Tests for hv.c for warnings.t

Makefile.SH

@@ -538,7 +538,7 @@ h = $(h1) $(h2) $(h3) $(h4) $(h5) $(h6)
 
 c1 = av.c scope.c op.c peep.c doop.c doio.c dump.c gv.c hv.c mg.c reentr.c mro_core.c perl.c
 c2 = perly.c pp.c pp_hot.c pp_ctl.c pp_sys.c utf8.c sv.c
-c3 = taint.c toke.c util.c deb.c run.c builtin.c universal.c pad.c globals.c keywords.c
+c3 = taint.c toke.c util.c deb.c run.c builtin.c universal.c class.c pad.c globals.c keywords.c
 c4 = perlio.c numeric.c mathoms.c locale.c pp_pack.c pp_sort.c caretx.c dquote.c time64.c
 c5 = regcomp.c regcomp_debug.c regcomp_invlist.c regcomp_study.c regcomp_trie.c regexec.c
 c6 = $(mallocsrc)
@@ -559,7 +559,7 @@ c = $(c_base) miniperlmain.c $(mini_only_src)
 obj1  = $(mallocobj) gv$(OBJ_EXT) toke$(OBJ_EXT) perly$(OBJ_EXT) pad$(OBJ_EXT)
 obj2  = regcomp$(OBJ_EXT) regcomp_debug$(OBJ_EXT) regcomp_invlist$(OBJ_EXT) regcomp_study$(OBJ_EXT) regcomp_trie$(OBJ_EXT)
 obj3  = regexec$(OBJ_EXT) dump$(OBJ_EXT) util$(OBJ_EXT) mg$(OBJ_EXT) reentr$(OBJ_EXT) mro_core$(OBJ_EXT)
-obj4  = keywords$(OBJ_EXT) builtin$(OBJ_EXT)
+obj4  = keywords$(OBJ_EXT) builtin$(OBJ_EXT) class$(OBJ_EXT)
 obj5  = hv$(OBJ_EXT) av$(OBJ_EXT) run$(OBJ_EXT) pp_hot$(OBJ_EXT) sv$(OBJ_EXT) pp$(OBJ_EXT)
 obj6  = scope$(OBJ_EXT) pp_ctl$(OBJ_EXT) pp_sys$(OBJ_EXT) peep$(OBJ_EXT)
 obj7  = doop$(OBJ_EXT) doio$(OBJ_EXT) utf8$(OBJ_EXT) taint$(OBJ_EXT)