RakuAST: add preliminary support for =alias / A<>

- add $*DOC-ALIASES dynamic variable, contains has with mapping
  of lemmas and their content
- add handling of =alias to ::Doc::Block.from-paragraphs
  The first atom is the lemma, the rest is the content
- add handling of A<> to set the contents of the lemma as the
  meta of the ::Doc::Markup
- add =alias and A<> handling to RakuDoc::To::Text (as in: don't
  render 'alias' blocks, and use the meta for rendering A<>

lib/RakuDoc/To/Text.rakumod

@@ -71,6 +71,7 @@ my multi sub rakudoc2text(RakuAST::Node:D $ast --> Str:D) {
 # the general handler, with specific sub-actions
 my multi sub rakudoc2text(RakuAST::Doc::Block:D $ast --> Str:D) {
     given $ast.type {
+        when 'alias'         { ''                 }
         when 'code'          { code2text($ast)    }
         when 'comment'       { ''                 }
         when 'config'        { ''                 }
@@ -139,7 +140,10 @@ my multi sub rakudoc2text(RakuAST::Doc::Markup:D $ast --> Str:D) {
     else {
         my str $text = $ast.atoms.map(&rakudoc2text).join;
 
-        if $letter eq 'L' {
+        if $letter eq 'A' {
+            rakudoc2text $ast.meta.head
+        }
+        elsif $letter eq 'L' {
             $text = colored($text, 'underline');
 
             # remember the URL as a note

src/Raku/Grammar.nqp

@@ -580,6 +580,9 @@ grammar Raku::Grammar is HLL::Grammar does Raku::Common {
         # into its statement list.
         my $*DOC-BLOCKS-COLLECTED := [];
 
+        # RakuDoc aliases (=alias -> A<>) collected so far
+        my $*DOC-ALIASES := {};
+
         # Any resolver that exists outside this grammar: usually this is the
         # resolver that is active whenever code is being EVALled inside BEGIN
         # block, which would create a new resolver and put it in $*R.

src/Raku/ast/doc-block.rakumod

@@ -125,7 +125,11 @@ class RakuAST::Doc::Block
             !! nqp::list);
         Nil
     }
-    method add-paragraph($paragraph) { nqp::push($!paragraphs, $paragraph) }
+    method add-paragraph($paragraph, :$at-start) {
+        $at-start
+          ?? nqp::unshift($!paragraphs, $paragraph)
+          !! nqp::push(   $!paragraphs, $paragraph)
+    }
     method paragraphs() { self.IMPL-WRAP-LIST($!paragraphs) }
 
     method visit-children(Code $visitor) {

src/core.c/RakuAST/Fixups.pm6

@@ -380,6 +380,14 @@ augment class RakuAST::Doc::Markup {
                 }
             }
         }
+        elsif $letter eq 'A' {
+            my $aliases := $*DOC-ALIASES;
+            unless nqp::istype($aliases,Failure) {
+                if nqp::atkey($aliases,self.atoms.head) -> $alias {
+                    self.set-meta($alias);
+                }
+            }
+        }
     }
 
     # flatten this markup recursively
@@ -754,18 +762,27 @@ augment class RakuAST::Doc::Block {
 
         elsif $type eq 'alias' {
             my @parts = @paragraphs;
-            # first line is the lemma, separate that
-            @parts.splice(0,1,@parts.head.split(/ \s+ /,2));
 
-            # lemma does not allow markup
-            $block.add-paragraph(@parts.shift);
+            # separate first word: lemma does not allow markup
+            @parts.splice(0,1,@parts.head.split(/ \s+ /,2));
+            my str $lemma = @parts.shift;
 
             # add rest with possible markup
             $block.add-paragraph(
               nqp::istype($_,Str)
                 ?? RakuAST::Doc::Paragraph.from-string($_)
                 !! $_
             ) for @parts;
+
+            # collect alias info if being collected
+            my $aliases := $*DOC-ALIASES;
+            nqp::bindkey(
+              $aliases,
+              $lemma,
+              nqp::clone(nqp::getattr($block.paragraphs,List,'$!reified'))
+            ) unless nqp::istype($aliases,Failure);
+
+            $block.add-paragraph($lemma, :at-start);
         }
 
         elsif $type eq 'defn' {