compiler: Reworked Preprocessor To Support Better Error Messages

Can now support showing the trail of includes in an error
message.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Sources/Common/Error.swift

@@ -55,8 +55,16 @@ public struct ErrorWithLocation: Errorable, Equatable, CustomStringConvertible {
 
     let prior_snipped = prior.trimmingPrefix(["\n"])
     let after_snipped = after.prefix { $0 != "\n" }
+    let include_list = fp.reversed().map {
+      let at = if let whence = $0.whence {
+        " included at position \(whence) in"
+      } else {
+        ""
+      }
+      return $0.path.string + at
+    }.joined(separator: " ")
     return formatter.formatWithStyle("Error: ", Style(StyleColor.Red))
-      + "In \(fp), there was an error: \n..." + prior_snipped
+      + "In \(include_list), there was an error: \n..." + prior_snipped
       + formatter.formatWithStyle(source, Style(.none, [StyleFormat.Underline])) + after_snipped
       + "...\n"
       + self._msg