Upgrade memmap2 to latest version. (#232)

Signed-off-by: Nick Hallahan <nick@theoutpost.io>

Author: hallahan

flatdata-rs/lib/Cargo.toml

@@ -12,7 +12,7 @@ edition = "2018"
 
 [dependencies]
 diff = "0.1.11"
-memmap2 = "0.2.0"
+memmap2 = "0.5.7"
 tar = { version = "0.4.38", optional = true }
 walkdir = "2.2.9"