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.
Every code example is now fully runnable and testable. As a result, all
examples are now tested and include imports. Relevant imports are shown
by default. Code examples can be expanded to show all imports.
Fixes#432.
