Verify data table sizes with table_width and assert_table_length macros

This was discussed in #706

It also uncovered some off-by-one issues with defining some constants.

A few structs now use rsreset/_RS to define their offset constants, as discussed in #739

macros/asserts.asm

@@ -0,0 +1,20 @@
+; Macros to verify assumptions about the data or code
+
+table_width: MACRO
+CURRENT_TABLE_WIDTH = \1
+if DEF(CURRENT_TABLE_START)
+PURGE CURRENT_TABLE_START
+endc
+if _NARG == 2
+CURRENT_TABLE_START EQUS "\2"
+else
+CURRENT_TABLE_START EQUS "._table_width\@"
+CURRENT_TABLE_START:
+endc
+ENDM
+
+assert_table_length: MACRO
+x = \1
+	assert x * CURRENT_TABLE_WIDTH == @ - CURRENT_TABLE_START, \
+		"{CURRENT_TABLE_START}: expected {d:x} entries, each {d:CURRENT_TABLE_WIDTH} bytes"
+ENDM

macros/scripts/battle_commands.asm

@@ -180,6 +180,7 @@ ENDM
 	command supereffectivelooptext  ; ad
 	command startloop               ; ae
 	command curl                    ; af
+NUM_EFFECT_COMMANDS EQU const_value - 1
 
 	const_def -1, -1
 	command endmove                 ; ff