Implict Let-nil to replace implicit Begin

Proposal

Certain forms, such as case, cond, when, and unless allow for multiple expressions to all follow one after another in certain parts of their syntax. These expressions are currently wrapped implicitly, in a begin form. This proposal suggests that we change this to an implicit LET-NIL wrapping.

Rationale

While preserving backwards compatibility, this change allows for internal definitions to occur conveniently within certain expressions, where they would otherwise not be allowed. This suits some programmer's styles, and makes some things convenient without breaking anything else.