Enable indexmap std feature

See:
* petgraph/petgraph#398
* indexmap-rs/indexmap#151

Author: alixinne

glslt/Cargo.toml

@@ -26,7 +26,7 @@ sha-1 = "0.10"
 petgraph = "0.6"
 bimap = "0.6"
 log = "0.4"
-indexmap = "1.8"
+indexmap = { version = "1.8", features = ["std"] }
 
 [dependencies.pyo3]
 version = "0.15"