Add string_set buildfile value type

This exposes the std::set<std::string> type to buildfiles.

New functions:

  $size(<string-set>)

Subscript returns true if the value is present and false otherwise (so
it is mapped to std::set::contains()). For example:

set = [string_set] a b c

if ($set[b])
  ...

Note that append (+=) and prepend (=+) have the same semantics
(std::set::insert()). For example:

set = [string_set] a b
set += c b              # a b c
set =+ d b              # a b c d

Example of iteration:

set = [string_set] a b c
for k: $set
  ...

1 files changed, 2 insertions, 1 deletions

diff --git a/tests/function/string/testscript b/tests/function/string/testscript
index a363cc3..96f5c52 100644
--- a/tests/function/string/testscript
+++ b/tests/function/string/testscript
@@ -46,7 +46,8 @@
   $* <'print $size([string] abc)'             >'3' : basics
   $* <'print $size([string] )'                >'0' : zero
   $* <'print $size([strings] a b c)'          >'3' : strings
-  $* <'print $size([string_map] a@1 b@2 c@3)' >'3' : string_map
+  $* <'print $size([string_set] a b b)'       >'2' : string-set
+  $* <'print $size([string_map] a@1 b@2 b@3)' >'2' : string-map
 }
 
 : find