Kernel syntax for when, unless, let/ec, and cond

[jesboat]

Note: the diff for this PR is best viewed with whitespace changes hidden.

Commit log:

racket/private: new raise-syntax-error-{if,unless} helpers

There is a very common pattern in the implementation of low-level macros:

(if something
    (void)
    (raise-syntax-error #f constant-string local-variable))

Introduce the appropriate abstraction to handle them.

This does not introduce uses; rather, uses will be added as more of the core syntax is centralized and some of the implementations are slightly modernized. See #5406 for more on that effort.

racket/private/define-et-al: use kernel syntax in when/unless

This is quite straightforward; mostly it's just changing a cond where the first pile of branches do error checking to a handful of raise-syntax-error-if and raise-syntax-error-unless invocations. I also took advantage of the opportunity to improve some error messages, going from:

bad syntax
missing test expression and body
missing body

to

bad syntax     [when used as an identifier macro or an improper list]
bad syntax (missing test expression and body)
bad syntax (missing body)

This is part of a larger effort to centralize and modernize some core syntax constructs; see #5406 for more on that.

racket/private/define-et-al: use kernel syntax for let/ec

Quite straightforward. This also improves the error messages from

bad syntax

to

bad syntax     [when used as an identifier macro or an improper list]
bad syntax (missing identifier and body)
bad syntax (missing body)

This is part of a larger effort to centralize and modernize some core syntax constructs; see #5406 for more on that.

racket/private/cond: modify cond to use kernel syntax only

Minimal modifications to the cond implementation to only use kernel syntax. Note that it still uses non-kernel functions.

This is part of a larger effort to centralize and modernize some core syntax constructs; see #5406 for more on that.