Merge branch 'master' of https://github.com/mthom/rusty-wam

Merge with downcast update.

diff --cc Cargo.toml
index e104be8693c25bc14b024bb250ed9e11d3627320,343e43515c990347516e18b5ba8a895471f8ddba..fa7e82432ff75dc9914ef1b9a4f2126c3cc7a6e0
--- 1/Cargo.toml
--- 2/Cargo.toml
+++ b/Cargo.toml
@@@ -11,10 -11,10 +11,10 @@@ default = ["readline_rs_compat"
  
  [dependencies]
  cfg-if = "0.1.7"
- downcast = "0.9.1"
+ downcast = "0.10.0"
  num = "0.2"
  ordered-float = "0.5.0"
 -prolog_parser = "0.8.3"
 +prolog_parser = "0.8.4"
  readline_rs_compat = { version = "0.1.7", optional = true }
  ref_thread_local = "0.0.0"