Header cleanup (#6993)

* Drop compatibility definitions for R < 3.4

SET_GROWABLE_BIT appeared in 3.4.0, which we now depend upon.
R_(Calloc|Realloc|Free) seem to have existed for a long time and
definitely were available in 3.4 (per #6380).

* Drop R-devel (pre 4.5.0) compatibility definitions

During the R-4.5.0 development cycle, R introduced a number of entry
points which we were required to use, necessitating checks for SVN
revision numbers for compatibility with older versions of R-devel.

Since R-4.5.0 has been released, remove the checks.

* Rf_isNull -> isNull for <3.5 compatibility

Author: aitap

src/assign.c

@@ -215,7 +215,7 @@ SEXP setdt_nrows(SEXP x)
      *   many operations still work in the presence of NULL columns and it might be convenient
      *   e.g. in package eplusr which calls setDT on a list when parsing JSON. Operations which
      *   fail for NULL columns will give helpful error at that point, #3480 and #3471 */
-    if (Rf_isNull(xi)) continue;
+    if (isNull(xi)) continue;
     if (Rf_inherits(xi, "POSIXlt")) {
       error(_("Column %d has class 'POSIXlt'. Please convert it to POSIXct (using as.POSIXct) and run setDT() again. We do not recommend the use of POSIXlt at all because it uses 40 bytes to store one date."), i+1);
     }

src/data.table.h

@@ -12,15 +12,8 @@
 #  define COMPLEX_RO COMPLEX
 #  define RAW_RO RAW
 #  define LOGICAL_RO LOGICAL
-#  define R_Calloc(x, y) Calloc(x, y)         // #6380
-#  define R_Realloc(x, y, z) Realloc(x, y, z)
-#  define R_Free(x) Free(x)
 #endif
-#if R_VERSION < R_Version(3, 4, 0)
-#  define SET_GROWABLE_BIT(x)  // #3292
-#endif
-// TODO: remove the `R_SVN_VERSION` check when R 4.5.0 is released (circa Apr. 2025)
-#if R_VERSION < R_Version(4, 5, 0) || R_SVN_REVISION < 86702
+#if R_VERSION < R_Version(4, 5, 0)
 #  define isDataFrame(x) isFrame(x) // #6180
 #endif
 #include <Rinternals.h>
@@ -44,8 +37,7 @@
 /* we mean the encoding bits, not CE_NATIVE in a UTF-8 locale */
 #define IS_UTF8(x)  (getCharCE(x) == CE_UTF8)
 #define IS_LATIN(x) (getCharCE(x) == CE_LATIN1)
-// TODO: remove the `R_SVN_VERSION` check when R 4.5.0 is released (circa Apr. 2025)
-#if R_VERSION < R_Version(4, 5, 0) || R_SVN_REVISION < 86789
+#if R_VERSION < R_Version(4, 5, 0)
 # define IS_ASCII(x) (LEVELS(x) & 64)
 #else
 # define IS_ASCII(x) (Rf_charIsASCII(x)) // no CE_ASCII