Use constants for trade text indexes

[Rangi42]

@Narishma-gb I based this on your pokegreen constants mentioned in #493 (thanks!). Some changes:

• The constants are defined directly in the engine file, since they're not needed anywhere else (this is in keeping with other locally-used constants, e.g. https://github.com/pret/pokecrystal/blob/master/engine/pokemon/stats_screen.asm)
• Used an assert instead of a comment for the xor a (sometimes there are xor a ; FOO comments when FOO is obviously some null/default value, but TRADETEXT_WANNA_TRADE isn't that obvious)
• Added table size assertions.

[SatoMew]

For reference and additional context, see #500.

[Narishma-gb]

Big fan of this! Perhaps capitalize assert, as is standard on pokered?