Allow {symbol} interpolation outside of strings

Fixes #629

Closes #631

test/asm/interpolation.asm

@@ -0,0 +1,15 @@
+SECTION "Test", ROM0
+
+NAME equs "ITEM"
+FMT equs "d"
+ZERO_NUM equ 0
+ZERO_STR equs "0"
+; Defines INDEX as 100
+INDEX = 1{ZERO_STR}{{FMT}:ZERO_NUM}
+; Defines ITEM_100 as "\"hundredth\""
+{NAME}_{d:INDEX} equs "\"hundredth\""
+; Prints "ITEM_100 is hundredth"
+PRINTT STRCAT("{NAME}_{d:INDEX} is ", {NAME}_{d:INDEX}, "\n")
+; Purges ITEM_100
+PURGE {NAME}_{d:INDEX}
+ASSERT !DEF({NAME}_{d:INDEX})

test/asm/interpolation.out

@@ -0,0 +1 @@
+ITEM_100 is hundredth