Fix {} escapes in inline I6

This was never lexed correctly, until now:

    To print (N - number): (- print (number) {N}; -).
Support inline I6 use options and routine endings

* Inline use option:

        Use maximum leaves of at least 69105 translates as (-
        Constant MAX_LEAVES = {N};
        -).

* Routine termination within an inline phrase (supported, but not
  encouraged):

        To end prematurely: (-
        ];
        [ Unused{-advance-counter:unused};
        -).
Fix inline invocation commands

Invocation commands, like {-open-brace}, were being shadowed by local
variables, like {x}.
Lex save and restore as keywords
Fix parentheses in I6 assembly

`sp` was lexed, properly, as a pseudokeyword here:

    @je (sp) 1 ?eq;

but not here:

    @je 1 (sp) ?eq;

Parentheses in assembly are only for grouping, not for function
application.
Fix noninitial I6T inclusions and other I7 woes

All I6T inclusions took their inlinedness from the first I6T inclusion.
Now, each inclusion is considered independently.

Use option I6T interprets both braced I7 and line-initial at signs now.

@c is allowed to have text following it on the same line, just like @p.
Remove support for line-initial assembly in @p

It did not lex well, and besides, it is undocumented and probably a
mistake in the language.
Add example.{i6t,i7x,inf,ni}
Fix I7 extension documentation headings
Fix I6 assembly

Prior changes to fix `sp` broke operators after parentheses in
assembly. The lexing of assembly now has a better overall design.