Hello,

The attached patch adds the pattern `cl-lambda` for Pcase, which works 
like `cl-destructuring-bind`. There are two differences with the lambda 
lists:

1. It does not support `&environment`
2. Without `&allow-other-keys` or `:allow-other-keys t`, the pattern 
will fail to match if their are unmatched keys in EXPVAL, but it does 
not throw an error.

The variable that would be bound in the lambda list can be Pcase 
patterns themselves, with two exceptions:

1. Using a sub-pattern as the cdr of a dotted list, like "(cl-lambda (a 
. `(,b . ,c))" doesn't work, since the pattern won't always look like a 
dotted list.
2. For constructs that use a sub-list to provide additional values, such 
as `&optional`, `&key`, and `&aux`, the sub-pattern only works inside 
the sub-list.  For example, one could do "(cl-lambda (&optional (`(,a 
,b))" but not "(cl-lambda (&optional `(,a ,b)))".

The pattern is useful when one wants to combine the features of `pcase` 
and `cl-destructuring-bind`, such combining the optional values with the 
`pred` or `guard` patterns.

Thank you.