Enable multiple heap types in a single multicall

[danielbate]

When instantiating a MultiCallInvocationScope where more than one of the functions returns a heap type we currently throw the following error:

fuels-ts/packages/program/src/functions/multicall-scope.ts

Lines 56 to 61 in c9597f9

	if (++numberOfHeaps > 1) {
	throw new FuelError(
	ErrorCode.INVALID_MULTICALL,
	'A multicall can have only one call that returns a heap type.'
	);
	}

V1 encoding supports multiple heap type returns as it concatenates all returned byte date in the return data receipt. Rather than across multiple receipts, as required in V1.

This should be supported in the V1 implementation and proved with tests. We also have a number of heap multicall tests that should be enabled.