New API uses variable name as record label.

[nsaunders]

Since originally publishing this library, I came to learn that row labels in PureScript don't have to be camel-case. Considering this, there isn't much point in this library attempting to map environment variable names. Instead, we might as well just use the environment variable name as the label, and allow the client code to map to camel-case if needed, hence this PR.

This is a breaking change, of course, but at least client code is simplified...

Before

TypedEnv.fromEnv (Proxy :: _ (foo :: String <: "FOO"))

After

TypedEnv.fromEnv (Proxy :: _ ("FOO" :: String))

Before

case TypedEnv.fromEnv (Proxy :: _ (foo :: String <: "FOO")) of
  Left err ->
    -- ...
  Right { foo } ->
    -- ....

After

case TypedEnv.fromEnv (Proxy :: _ ("FOO" :: String)) of
  Left err ->
    -- ...
  Right { "FOO": foo } ->
    -- ....