feat: creating new mx_ suite of macros!

also adding new mx_getcode macro

tests/base/mf_getuniquefileref.test.sas

@@ -0,0 +1,36 @@
+/**
+  @file
+  @brief Testing mf_getuniquefileref macro
+  @details To test performance you can also use the following macro:
+
+      %macro x(prefix);
+      %let now=%sysfunc(datetime());
+      %do x=1 %to 1000;
+        %let rc=%mf_getuniquefileref(prefix=&prefix);
+      %end;
+      %put %sysevalf(%sysfunc(datetime())-&now);
+      %mend;
+      %x(_)
+      %x(0)
+
+  <h4> SAS Macros </h4>
+  @li mf_getuniquefileref.sas
+  @li mp_assert.sas
+
+**/
+
+%mp_assert(
+  iftrue=(
+    "%substr(%mf_getuniquefileref(prefix=0),1,1)"="#"
+  ),
+  desc=Checking for a natively assigned fileref,
+  outds=work.test_results
+)
+
+%mp_assert(
+  iftrue=(
+    "%substr(%mf_getuniquefileref(),1,1)"="_"
+  ),
+  desc=Checking for a default fileref,
+  outds=work.test_results
+)