Add nulls, and more newdecl and type system features.

New null_t type has a constant "null", which can be assigned to objects.
Methodmaps can now be "nullable", accepting null_t types.
The delete keyword now zaps non-const lvalue inputs.
Fixed some newdecl arguments not accepting references.
Allow functag matching to coerce arguments and return values.
Allow functags to accept new-style types as a return value.

sourcepawn/compiler/sc.h

@@ -411,6 +411,8 @@ enum {
   tMETHODMAP,
   tNATIVE,
   tNEW,
+  tNULL,
+  tNULLABLE,
   tOBJECT,
   tOPERATOR,
   tPUBLIC,
@@ -675,6 +677,7 @@ SC_FUNC cell array_totalsize(symbol *sym);
 SC_FUNC int matchtag_string(int ident, int tag);
 SC_FUNC int checktag_string(value *sym1, value *sym2);
 SC_FUNC int checktags_string(int tags[], int numtags, value *sym1);
+SC_FUNC int lvalexpr(svalue *sval);
 
 /* function prototypes in SC4.C */
 SC_FUNC void writeleader(symbol *root);
@@ -699,6 +702,7 @@ SC_FUNC void copyarray(symbol *sym,cell size);
 SC_FUNC void fillarray(symbol *sym,cell size,cell value);
 SC_FUNC void ldconst(cell val,regid reg);
 SC_FUNC void moveto1(void);
+SC_FUNC void move_alt(void);
 SC_FUNC void pushreg(regid reg);
 SC_FUNC void pushval(cell val);
 SC_FUNC void popreg(regid reg);
@@ -923,6 +927,7 @@ SC_VDECL int pc_tag_string;   /* global String tag */
 SC_VDECL int pc_tag_void;     /* global void tag */
 SC_VDECL int pc_tag_object;   /* root object tag */
 SC_VDECL int pc_tag_bool;     /* global bool tag */
+SC_VDECL int pc_tag_null_t;   /* the null type */
 SC_VDECL int pc_anytag;       /* global any tag */
 SC_VDECL int glbstringread;	  /* last global string read */
 SC_VDECL int sc_require_newdecls; /* only newdecls are allowed */