Revamp configuration.

This commit completely overhauls Rocket's configuration systems, basing
it on the new Figment library. It includes many breaking changes
pertaining to configuration. They are:

  * "Environments" are replaced by "profiles".
  * 'ROCKET_PROFILE' takes the place of 'ROCKET_ENV'.
  * Profile names are now arbitrary, but 'debug' and 'release' are given
    special treatment as default profiles for the debug and release
    compilation profiles.
  * A 'default' profile now sits along-side the meta 'global' profile.
  * The concept of "extras" is no longer present; users can extract any
    values they want from the configured 'Figment'.
  * The 'Poolable' trait takes an '&Config'.
  * The 'secrets' feature is disabled by default.
  * It is a hard error if 'secrets' is enabled under the 'release'
    profile and no 'secret_key' is configured.
  * 'ConfigBuilder' no longer exists: all fields of 'Config' are public
    with public constructors for each type.
  * 'keep_alive' is disabled with '0', not 'false' or 'off'.
  * Inlined error variants into the 'Error' structure.
  * 'LoggingLevel' is now 'LogLevel'.
  * Limits can now be specified in SI units: "1 MiB".

The summary of other changes are:

  * The default config file can be configured with 'ROCKET_CONFIG'.
  * HTTP/1 and HTTP/2 keep-alive configuration is restored.
  * 'ctrlc' is now a recognized config option.
  * 'serde' is now a core dependency.
  * TLS misconfiguration errors are improved.
  * Several example use '_' as the return type of '#[launch]' fns.
  * 'AdHoc::config()' was added for simple config extraction.
  * Added more documentation for using 'Limits'.
  * Launch information is no longer treated specially.
  * The configuration guide was rewritten.

Resolves #852.
Resolves #209.
Closes #1404.
Closes #652.

contrib/codegen/src/database.rs

@@ -71,13 +71,13 @@ pub fn database_attr(attr: TokenStream, input: TokenStream) -> Result<TokenStrea
     let databases = quote_spanned!(span => ::rocket_contrib::databases);
     let request = quote!(::rocket::request);
 
-    let generated_types = quote_spanned! { span =>
+    let request_guard_type = quote_spanned! { span =>
         /// The request guard type.
         #vis struct #guard_type(#databases::Connection<Self, #conn_type>);
     };
 
     Ok(quote! {
-        #generated_types
+        #request_guard_type
 
         impl #guard_type {
             /// Returns a fairing that initializes the associated database
@@ -110,8 +110,8 @@ pub fn database_attr(attr: TokenStream, input: TokenStream) -> Result<TokenStrea
         impl<'a, 'r> #request::FromRequest<'a, 'r> for #guard_type {
             type Error = ();
 
-            async fn from_request(request: &'a #request::Request<'r>) -> #request::Outcome<Self, ()> {
-                <#databases::Connection<Self, #conn_type>>::from_request(request).await.map(Self)
+            async fn from_request(req: &'a #request::Request<'r>) -> #request::Outcome<Self, ()> {
+                <#databases::Connection<Self, #conn_type>>::from_request(req).await.map(Self)
             }
         }
     }.into())

contrib/codegen/tests/ui-fail-nightly/database-syntax.stderr

@@ -1,4 +1,4 @@
-error: unexpected end of input, expected literal
+error: unexpected end of input, expected string literal
  --> $DIR/database-syntax.rs:6:1
   |
 6 | #[database]

contrib/codegen/tests/ui-fail-stable/database-syntax.stderr

@@ -1,4 +1,4 @@
-error: unexpected end of input, expected literal
+error: unexpected end of input, expected string literal
  --> $DIR/database-syntax.rs:6:1
   |
 6 | #[database]

contrib/lib/Cargo.toml

@@ -14,7 +14,10 @@ edition = "2018"
 [features]
 # Internal use only.
 templates = ["serde", "serde_json", "glob", "notify"]
-databases = ["r2d2", "tokio/blocking", "tokio/rt-threaded", "rocket_contrib_codegen/database_attribute"]
+databases = [
+    "serde", "r2d2", "tokio/blocking", "tokio/rt-threaded",
+    "rocket_contrib_codegen/database_attribute"
+]
 
 # User-facing features.
 default = ["json", "serve"]