sinput.adb: Minor code cleanup.

sinput.adb: Minor code cleanup.

2017-04-27  Bob Duff  <duff@adacore.com>

	* sinput.adb: Minor code cleanup.
	* namet.adb (Append): Create faster versions of
	Append(String) and Append(Name_Id) by using slice assignment
	instead of loops.
	* sem_util.adb (In_Instance): Speed this up by removing
	unnecessary tests; Is_Generic_Instance is defined for all
	entities.
	* sem_util.ads, sem_util.adb (In_Parameter_Specification):
	Remove unused function.
	* alloc.ads (Nodes_Initial): Use a much larger value, because
	the compiler was spending a lot of time copying the nodes table
	when it grows. This number was chosen in 1996, so is rather out
	of date with current memory sizes. Anyway, it's virtual memory.
	Get rid of Orig_Nodes_...; use Node_... instead.
	* atree.adb (Lock): Do not release the Nodes tables; it's a
	waste of time.
	Orig_Nodes_ ==> Nodes_
	* nlists.adb: Orig_Nodes_ ==> Nodes_
	* g-table.adb: Remove unused "with" clause.
	* g-table.ads, table.ads: Remove Big_Table_Type, which should
	not be used by clients.
	* g-dyntab.adb (Last_Allocated): New function
	to encapsulate T.P.Last_Allocated, which I'm thinking of changing.

From-SVN: r247335