Finding abuse of discretion in attempted deportation of an alien who had relied on agency regulation's definition of "investor" rather than an adjudicative definition enunciated after she had made her initial investment
Holding that where a policy "may be stated and applied as a general rule" rather than by case-by-case determinations, it should be announced via rule-making, and that policymaking via adjudication should be struck down when it represents "a circumvention of rulemaking procedures"