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/pseudo.c

@@ -1967,6 +1967,7 @@ static CtrlDesc CtrlCmdTab [] = {
     { ccNone,           DoUnexpected    },      /* .ADDRSIZE */
     { ccNone,           DoAlign         },
     { ccNone,           DoASCIIZ        },
+    { ccNone,           DoUnexpected    },      /* .ASIZE */
     { ccNone,           DoAssert        },
     { ccNone,           DoAutoImport    },
     { ccNone,           DoUnexpected    },      /* .BANK */
@@ -2041,6 +2042,7 @@ static CtrlDesc CtrlCmdTab [] = {
     { ccNone,           DoIncBin        },
     { ccNone,           DoInclude       },
     { ccNone,           DoInterruptor   },
+    { ccNone,           DoUnexpected    },      /* .ISIZE */
     { ccNone,           DoUnexpected    },      /* .ISMNEMONIC */
     { ccNone,           DoInvalid       },      /* .LEFT */
     { ccNone,           DoLineCont      },