lib/macro/internal: Only use alloc instead of std

Fix #47. The problem is that for stable compilers older than 1.51, we used to
support no-std alloc environments, but not no-alloc environments. For compilers
1.51 and later, we additionally support no-alloc environments using resolver =
"2" in Cargo.toml of the end binary.

Author: ia0

lib/macro/internal/Cargo.toml

@@ -13,5 +13,5 @@ include = ["Cargo.toml", "LICENSE", "README.md", "src/lib.rs"]
 proc-macro = true
 
 [dependencies]
-data-encoding = { version = "2.3", path = "../.." }
+data-encoding = { version = "2.3", path = "../..", default-features = false, features = ["alloc"] }
 syn = "1"