Otherwise we'll erroneously think that 'descriptor' ('descriptor.rb')
is a loaded feature when somebody requires 'descriptor.pb' ('descriptor.pb.rb'),
when in fact those are two completely unrelated files.

Elidable extensions include Ruby '.rb' files and shared libraries.

Ruby does some slightly magical things in 'require', e.g., letting
you refer to shared libraries by extensions that _would_ have been
valid on another platform ('libgit2.dll' works on Linux!) which would
be difficult to replicate precisely without a clear spec, so this change
makes the simplifying assumption that people won't use these extensions
for anything other than shared libraries.

Remove cast from const pointer to non-const pointer

Only strip elidable extensions to make short forms

Update minimum ruby version and fix rubocop

Don't break when Pathname is passed to LoadedFeaturesIndex

The old documentation was ambiguous and incorrect. `uname -v` makes it
clear that the Bootsnap cache won't be reused in a Dockerized
application when the host kernel changes.

Update documentation on os_version -> ruby_platform

Freezing the error message for LoadError triggers the
`can't modify frozen String` error at
https://github.com/rails/rails/blob/6-0-stable/activesupport/lib/active_support/dependencies.rb#L370
which `replace`s the error message

Unfreeze string literal

Previously, the `ruby_platform` header in the cache was calculated as a
hash of the `RUBY_PLATFORM` (e.g. x86_64-linux-gnu) with the `uname -v`
output (e.g. `#75-Ubuntu SMP Tue Oct 1 05:24:09 UTC 2019`).

However, using `uname -v` is a bit restrictive on Linux systems. It
makes it difficult for a containerized application to bundle the
Bootsnap cache with the application itself. For example, in Docker, the
host operating system determines the kernel version, so even though a
Docker image may be built with a specific base image, the output of
`uname -v` will change depending on where it's run. As a result, the
application will need to rebuild the Bootsnap cache every time unless a
persistent volume is used AND the kernel version is fixed. However, both
requirements can be difficult to guarantee, which negates any speed-up
that Bootsnap provides.

We observe that in a Linux system, the application binary interface
(ABI) is defined by the kernel version and glibc version. A change in
the kernel ABI requires a change to glibc. Instead of hashing the output
of `uname -v`, we hash the glibc version if it is defined by the
compiler.

Use glibc version in ruby_platform cache header if available