Use std::unordered_map and std::vector for sections

This allows us to control the order in which sections are iterated,
instead of it depending on the internals of `std::map`. (This order
is arbitrary, but should be deterministic regardless.)

test/asm/charmap-inheritance.asm

@@ -13,7 +13,7 @@ SECTION "test", ROM0
 	charmap "<FAR>",    $08
 
 	; At this point, enough nodes were allocated for 'foo' to be reallocated.
-	; Its value in the charmaps' std::map should have been updated too,
+	; Its value in the charmaps' `std::unordered_map` should have been updated too,
 	; so that usages of 'foo' will not segfault.
 
 	; This uses 'foo; by switching to it.

test/asm/sym-collision.asm

@@ -2,7 +2,7 @@ SECTION "Collision course", OAM[$FE00]
 
 ; All the following symbols used to collide with our custom hashmap,
 ; which at some point caused `PURGE` to malfunction with them.
-; We now use C++ `std::map` which reliably handles collisions.
+; We now use C++ `std::unordered_map` which reliably handles collisions.
 aqfj: ds 1 ; Give them different addresses
 cxje: ds 1
 dgsd: ds 1