WIP - Replace boost memory-mapped files with own implementation.

[junhaoliao]

References

When compiling the Zstd decompressor code with Emscripten for use in Log Viewer WebUI, it was identified that the code has dependency on Boost's memory-mapped files. That requires setting up extra compiling procedures - downloading Boost and also making modifications in Boost to avoid emcc complaining on missing some std::atomic implementations. However, the Boost dependencies can be replace with POSIX system calls.

In Emscripten compiled example code, with the replacement the speedup is > 30 ms.
 

Description

This is WIP. Criticism is welcomed:

1. Shall we abstract the changes into a class to avoid bloating the Decoder class with 3 extra members?
2. Shall we replace other Boost memory-mapped file usages in CLP core?

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1. Replace boost memory-mapped files with own implementation.

Validation performed

1. Built and ran unitTest target and passed all:

   ...
   All tests passed (95962 assertions in 51 test cases)
   

[LinZhihao-723]

We should use std::span (probably wrapping with std::optional) instead

[kirkrodrigues]

1. Shall we abstract the changes into a class to avoid bloating the Decoder class with 3 extra members?
2. Shall we replace other Boost memory-mapped file usages in CLP core?

If we do (2), then we should do (1). I think it would be a good idea to do both, but if we're time constrained, I think we could go with the current approach. If so, we can convert this to non-WIP and do a proper review.

[LinZhihao-723]

This PR has been formally implemented in #445