Option sets: a .o target for shared parameters

A named group of optional parameters, declared once and used by several
klammers, so a writer learns one vocabulary instead of a spelling per
klammer.  The "o" target is a pseudo-target beside "k": "k" declares a
klammer's interface and documents it, "o" declares an option interface and
documents it, and neither produces output for any target.

    @@caption_args.o :caption :number.bool true :caption_side.side
    : Arguments that define a caption for a block element @@

    @@code.k :filename @hpos_args :hpos left @ @caption_args :caption_side top @
    | text.literal : A source file displayed verbatim @@

A set is used only in the parameter list of a ".k" declaration -- the one
place a klammer's interface is declared once for all of its targets -- and
is resolved as that list is read.  Names and types come from the set; a
default may be overridden where it is used.  A klammer application in a
parameter list is now a definition-time error.

The SKS gains the sets caption_args and hpos_args (:hpos and :offset), and
@table, @image, @image_grid, @reference and @show gain .k declarations.  A
distance is no longer written as a position: :hpos 4em is rejected, and the
same layout is :hpos left :offset 4em.  Code listings are numbered by
default, like tables and figures.

New engine sources mac/option_set{,_registry}.{h,cpp}; tst/ ships two more
suites, option_set_test.sh and signature_test.sh (twelve in all).

 (from dev 34e536cb0329)

sks/target/html_util.py

@@ -193,27 +193,38 @@ def element_tag(element):
 def font_class(font):
     return {"r" : "", "i" : "ritalic", "t" : "monospace", "s" : "sanserif"}[font]
 
-def hpos_container(element, hpos):
+def hpos_container(element, hpos, offset=None):
     # Wrap element in its horizontal-position container.  hpos is left,
     # center, right, none, or a length, which becomes the left margin
     # (the tex counterparts are the \LTleft glue for tables and the
     # \hspace* in latex_util.caption_wrapper).
-    style = None
+    #
+    # offset insets the element from the margin hpos names, as padding on
+    # that side of the flex container: the left for "left", the right for
+    # "right".  A centered element has no such margin, so the offset is
+    # ignored there rather than being an error -- same rule as
+    # latex_util.caption_wrapper, which is the tex counterpart.  (It
+    # replaces the fixed 1em that the hpos_margin class used to add to
+    # every left- and right-placed element: that was this offset, hard
+    # coded, and only in html.)
+    styles = []
     if hpos not in ("left", "center", "right", "none"):
         length, _, _ = kutil.parse_length("html", hpos, 1)
-        style = f"margin-left: {length}"
+        styles.append(f"margin-left: {length}")
         hpos = "indent"
+    if offset and offset not in ("0", "0pt") and hpos in ("left", "right", "indent"):
+        length, _, _ = kutil.parse_length("html", offset, 1)
+        side = "right" if hpos == "right" else "left"
+        styles.append(f"padding-{side}: {length}")
     result = E("div").cls("hpos_" + hpos).body(element)
-    if style:
-        result.attr("style", style)
-    if hpos not in ("center", "indent"):
-        result.cls("hpos_margin")
+    if styles:
+        result.attr("style", "; ".join(styles))
     return result
 
 
 def add_caption(element, caption_label, number, caption_text,
                 font_symbol="i", hpos="center", side="bottom", as_string=True,
-                font_size=.9, width=None, max_width=None):
+                font_size=.9, width=None, max_width=None, offset=None):
     tag = element_tag(element)
     # Caption
     caption = ""
@@ -263,7 +274,7 @@ def add_caption(element, caption_label, number, caption_text,
             element.attr("style", f"max-width: {max_width}")
 
 
-    result = hpos_container(element, hpos)
+    result = hpos_container(element, hpos, offset)
 
     if number:
         result.cls("element_container")