API has been made stricter, layers are now managed with shared pointers,
so using them temporarily increased their reference counters.
Additionally, any s_layers map has been guarded by a read/write lock,
as concurrent write/reads to it were possible.
This excludes the second argument from template deduction.
Otherwise, it is required to manually cast the second argument to
the ConfigInfo type (because implicit conversions won't work).
e.g. to set the value for a ConfigInfo<std::string> from a string
literal, you'd need a ugly `std::string("yourstring")`.
Currently, a simple typo in the system name will trigger an assert
message that complains about a "programming error". This is not
user friendly and misleading.
So this changes GetSystemFromName to return an std::optional, which
allows for callers to check whether the system exists and handle
failures better.
Originally, Layer contained a std::map of Sections, which containted a std::map
containing the (key, value) pairs. Here we flattern this structure so that only
one std::map is required, reducing the number of indirections required and
vastly simplifying the code.
The Config::AddLoadLayer functions call Load on the layer
explicitly, but Load is already called in the constructor,
so they'd cause the loader's Load function to be called twice,
which is potentially expensive considering we have to read an INI
from the host filesystem.
This commit removes the Config::AddLoadLayer functions because
they don't appear to be necessary.
