Added .asize and .isize pseudo variables

These pseudo variables will return the size of the accumulator/index
in bits.

For the 65816 instruction set .ASIZE/.ISIZE will return either 8 or 16,
depending on the current size of the operand in immediate addressing
mode.

For all other CPU instruction sets, .ASIZE/.ISIZE will always return 8.

For example:

	; Reverse Subtract with Accumulator
	; A = memory - A
	.macro rsb param
		.if .asize = 8
			eor	#$ff
		.else
			eor	#$ffff
		.endif
		sec
		adc	param
	.endmacro

src/ca65/token.h

@@ -126,6 +126,7 @@ typedef enum token_t {
     TOK_ADDRSIZE,
     TOK_ALIGN,
     TOK_ASCIIZ,
+    TOK_ASIZE,
     TOK_ASSERT,
     TOK_AUTOIMPORT,
     TOK_BANK,
@@ -200,6 +201,7 @@ typedef enum token_t {
     TOK_INCBIN,
     TOK_INCLUDE,
     TOK_INTERRUPTOR,
+    TOK_ISIZE,
     TOK_ISMNEMONIC,
     TOK_LEFT,
     TOK_LINECONT,