Break up library into macros and map nested libraries for shared dependency with constant map macro.

macros/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "macros"
+version = "0.0.0"
+authors = ["paladin <christian.lincoln25@ic.ac.uk>"]
+description = "Procedural macros for Deimos"
+keywords = ["template", "proc_macro", "procmacro"]
+edition = "2018"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+quote = "1"
+proc-macro2 = "1.0"
+syn = "1.0"
+map = { path = "../map" }