Move the implementation into a consistent namespace libfsst.

[joka921]

The implementation of fsst as found in libfss[12].h/.cpp currently uses some very common class names like Symbol, Decoder, Encoder which are very likely to collide with similarly named classes in different libraries and then lead to very nasty violations of the one-definition-rule (ODR). This PR moves all those functions into a namespace libfsst:: to mitigate these problems.

An alternative approach would be to rename this types, e.g. FsstEncoder, FsstSymbol, etc., just tell me, if you prefer that approach.

Currently the formatting is very inconsistent, do you have a certain formatting style, or even better a configuration for an autoformatter liike clang-format?

[peterboncz]

thanks for the PR -- it is a good idea I think but currently there are very many whitespace changes unrelated to the proposed change. Could you please turn this into a PR with minimal hamming distance? Thanks in advance.