Allow LOAD FRAGMENT (#1552)

This was implemented in #736 but removed after discussion in #869.

Fixes #1537

test/link/load-fragment/section-fragment/a.asm

@@ -0,0 +1,8 @@
+SECTION FRAGMENT "rom", ROM0
+Part1::
+LOAD FRAGMENT "ram", WRAM0
+wPart1::
+	jr wPart1
+	jr wPart3
+ENDL
+Part1End::

test/link/load-fragment/section-fragment/b.asm

@@ -0,0 +1,4 @@
+SECTION FRAGMENT "rom", ROM0
+Part2::
+db "Hello world!"
+Part2End::

test/link/load-fragment/section-fragment/c.asm

@@ -0,0 +1,8 @@
+SECTION FRAGMENT "rom", ROM0
+Part3::
+LOAD FRAGMENT "ram", WRAM0
+wPart3::
+	jr wPart1
+	jr wPart3
+ENDL
+Part3End::

test/link/load-fragment/section-fragment/ref.out.map

@@ -0,0 +1,24 @@
+SUMMARY:
+	ROM0: 20 bytes used / 16364 free
+	WRAM0: 8 bytes used / 4088 free
+
+ROM0 bank #0:
+	SECTION: $0000-$0013 ($0014 bytes) ["rom"]
+	         $0000 = Part1
+	         $0004 = Part1End
+	         ; Next fragment
+	         $0010 = Part3
+	         $0014 = Part3End
+	         ; Next fragment
+	         $0004 = Part2
+	         $0010 = Part2End
+	EMPTY: $0014-$3fff ($3fec bytes)
+	TOTAL EMPTY: $3fec bytes
+
+WRAM0 bank #0:
+	SECTION: $c000-$c007 ($0008 bytes) ["ram"]
+	         $c000 = wPart1
+	         ; Next fragment
+	         $c004 = wPart3
+	EMPTY: $c008-$cfff ($0ff8 bytes)
+	TOTAL EMPTY: $0ff8 bytes

test/link/load-fragment/section-fragment/ref.out.sym

@@ -0,0 +1,9 @@
+; File generated by rgblink
+00:0000 Part1
+00:0004 Part1End
+00:0004 Part2
+00:0010 Part3
+00:0010 Part2End
+00:0014 Part3End
+00:c000 wPart1
+00:c004 wPart3