Convert LangID from u16 enum to tuple struct containing u16

Relax lang_id checks provided by enum so that any lang_id could be
used, not only values listed in the enum. These checks increased
the code size considerably without much benefit. Resolves #139.

Language identifiers like LangID::EN_US have a new type, but this
should not require source code changes in a code which uses them.

Provide two traits to convert LangID to and from u16 freely. This
implicitly provides TryFrom trait which previously was derived
using TryFromPrimitive.

There's no special constructor for LangID for values outside of
the provided constants, but it can be converted from any u16.

As TryFromPrimitive in no longer used, remove num_enum dependency.

Author: vitalyvb

Cargo.toml

@@ -12,7 +12,6 @@ repository = "https://github.com/rust-embedded-community/usb-device"
 [dependencies]
 defmt = { version = "0.3", optional = true }
 portable-atomic = { version = "1.2.0", default-features = false }
-num_enum = { version = "0.7.1", default-features = false }
 heapless = "0.8"
 
 [dev-dependencies]