Implement a '#' prefix for raw identifiers that may alias keywords (#1480)

* Implement a '#' prefix for raw identifiers that may alias keywords

* Review comments

* Disallow hashless raw identifiers in interpolations

* Run clang-format

test/asm/raw-identifiers.asm

@@ -0,0 +1,36 @@
+def #DEF equ 1
+def #def equ 2
+def #ghi equ 3
+export #def, #ghi
+
+def #align = 0
+def #rb rb #def
+
+MACRO #macro
+	println "\<#def> is not \<#DEF>"
+ENDM
+	#macro first, second
+	purge #macro
+	assert !def(#macro)
+
+section "section", rom0
+#section::
+	dw #section
+#.rom0:
+	db BANK(#section.rom0)
+#section.romx:
+	println "section.romx is in ", SECTION(.romx)
+
+def #sub equs "def"
+{#sub} #add equs "#"
+
+for #for, {{#add}{#sub}}
+	println "for == ", #for
+endr
+	assert #for == 2
+	assert !{#sub}(#FOR)
+
+	newcharmap #charmap, #main
+	charmap "#", $42
+	setcharmap #charmap
+	db "#"