Add `withFacet` template

Simplifies the injection of `getCurrentFacet`.

src/syndicate.nim

@@ -4,6 +4,7 @@ import std/[asyncdispatch, macros, options]
 import preserves
 import syndicate/[assertions, dataspaces, events, skeletons]
 
+export preserves.`%`
 export assertions.`?_`
 export assertions.`?*`
 export assertions.Observe
@@ -178,6 +179,20 @@ template spawn*(name: string; spawnBody: untyped): untyped =
     proc getCurrentFacet(): Facet {.inject, used.} = spawnFacet
     spawnBody
 
+template withFacet*(f: Facet; body: untyped): untyped =
+  ## Execute a Syndicate ``body`` using the ``Facet`` at ``f``.
+  runnableExamples:
+    import preserves, preserves/records
+    type Foo = ref object
+      facet: Facet
+      i: int
+    proc incAndAssert(foo: Foo) =
+      inc(foo.i)
+      withFacet foo.facet:
+        react: assert: initRecord("Foo", %foo.i)
+  proc getCurrentFacet(): Facet {.inject, used.} = f
+  body
+
 template syndicate*(ident, dataspaceBody: untyped): untyped =
   proc `ident`*(facet: Facet) =
     proc getCurrentFacet(): Facet {.inject, used.} = facet
@@ -185,6 +200,8 @@ template syndicate*(ident, dataspaceBody: untyped): untyped =
   when isMainModule:
     asyncCheck bootModule("", `ident`)
 
-template boot*(module: proc (facet: Facet) {.gcsafe.}) =
+type BootProc* = proc (facet: Facet) {.gcsafe.}
+
+template boot*(module: BootProc) =
   mixin getCurrentFacet
   module(getCurrentFacet())