Prevent defining invalid local labels

Fixes #913

test/asm/empty-local.err

@@ -0,0 +1,2 @@
+FATAL: empty-local.asm(5):
+    'Label.' is a nonsensical reference to an empty local label

test/asm/multiple-dots-local.asm

@@ -0,0 +1,7 @@
+SECTION "sec", ROM0
+
+Parent:
+Parent.heir:
+	db 0
+Parent...spare:
+	db 1

test/asm/multiple-dots-local.err

@@ -0,0 +1,2 @@
+FATAL: multiple-dots-local.asm(6):
+    'Parent...spare' is a nonsensical reference to a nested local label

test/asm/nested-local-reference.asm

@@ -0,0 +1,8 @@
+SECTION "sec", ROM0
+
+Parent:
+Parent.child:
+	db 0
+.grandchild:
+	db 1
+	dw Parent.child.grandchild

test/asm/nested-local-reference.err

@@ -0,0 +1,2 @@
+FATAL: nested-local-reference.asm(8):
+    'Parent.child.grandchild' is a nonsensical reference to a nested local symbol

test/asm/nested-local.asm

@@ -0,0 +1,7 @@
+SECTION "sec", ROM0
+
+Parent:
+Parent.child:
+	db 0
+Parent.child.grandchild:
+	db 1

test/asm/nested-local.err

@@ -0,0 +1,2 @@
+FATAL: nested-local.asm(6):
+    'Parent.child.grandchild' is a nonsensical reference to a nested local label

test/asm/remote-local-noexist.asm

@@ -4,4 +4,4 @@ Parent:
 .child:
 	db 0
 NotParent:
-	dw Parent.child.fail
+	dw Parent.orphan

test/asm/remote-local-noexist.err

@@ -1,2 +0,0 @@
-FATAL: remote-local-noexist.asm(7):
-    'Parent.child.fail' is a nonsensical reference to a nested local symbol

test/asm/sym-scope.err

@@ -3,7 +3,7 @@ ERROR: sym-scope.asm(4):
 ERROR: sym-scope.asm(5):
     Local label 'Nice.try' in main scope
 ERROR: sym-scope.asm(17):
-    Not currently in the scope of 'Parentheses.check'
+    Not currently in the scope of 'Parentheses'
 ERROR: sym-scope.asm(21):
-    Not currently in the scope of 'Parent.check'
+    Not currently in the scope of 'Parent'
 error: Assembly aborted (4 errors)!