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, 4 insertions, 0 deletions

diff --git a/tests/type/set/buildfile b/tests/type/set/buildfile
new file mode 100644
index 0000000..55b37bb
--- /dev/null
+++ b/tests/type/set/buildfile
@@ -0,0 +1,4 @@
+# file      : tests/type/set/buildfile
+# license   : MIT; see accompanying LICENSE file
+
+./: testscript $b