Reduce Tramp's memory usage

Reduce Tramp's memory usage

Construct Tramp syntax strings and regular expressions once instead
of every time they are used, and store them in alists keyed by Tramp
syntax.
* tramp.el (tramp-build-remote-file-name-spec-regexp)
(tramp-build-file-name-structure): New functions.
(tramp-prefix-format-alist, tramp-prefix-regexp-alist)
(tramp-method-regexp-alist)
(tramp-postfix-method-format-alist)
(tramp-postfix-method-regexp-alist)
(tramp-prefix-ipv6-format-alist, tramp-prefix-ipv6-regexp-alist)
(tramp-postfix-ipv6-format-alist)
(tramp-postfix-ipv6-regexp-alist)
(tramp-postfix-host-format-alist)
(tramp-postfix-host-regexp-alist)
(tramp-remote-file-name-spec-regexp-alist)
(tramp-file-name-structure-alist): New constants.
(tramp-lookup-syntax): New function.
(tramp-prefix-format, tramp-prefix-regexp, tramp-method-regexp)
(tramp-postfix-method-format, tramp-postfix-method-regexp)
(tramp-prefix-ipv6-format, tramp-prefix-ipv6-regexp)
(tramp-postfix-ipv6-format, tramp-postfix-ipv6-regexp)
(tramp-postfix-host-format, tramp-postfix-host-regexp)
(tramp-remote-file-name-spec-regexp, tramp-file-name-structure):
Use it.