Add Optimization Of Recursive Style Use To Avoid Re-parsing All Of Format

[rtillery]

Each level of fmtster formatting re-parses the format string. I will admit to not being an expert on whether the constexpr design of fmt::formatter<>::parse() causes much of this to be done at compile time, but I think it's possible that this will be less likely when Add Recursive Brace Support is done, so that formatting can more easily be done at run time. A method of passing the pre-parsed data (currently things like FmtsterBase::mStyleSetting, FmtsterBase::mTab, etc.) to avoid reparsing, if the values didn't change, would be helpful.

[rtillery]

With the work being done in https://github.com/HarmanFOSS/fmtster/tree/enhancement/27-new-JSON-style_union, the approach to this is coming into focus.

First, since the actual configuration structure is effectively a large integer, and there is a fmtster::XXXStyleHelper associated with the configuration to provide the expanded strings (et al), the efficiency sought here can be realized by storing an already initialized XXXStyleHelper and quickly associating it with a matching fmtster::XXXStyle(.value) object.

An interesting approach would be to maintain a std::map<fmtster::internal::XXX_VALUE_T, fmtster::internal::XXXStyleHelper>.

It will be necessary to investigate whether indexing would use the XXX_VALUE_T directly as desired and provide custom comparitor if necessary.

To keep the size of this from growing too large, an additional std::list<fmtster::internal::XXX_VALUE_T> could also be maintained for the same entries as the map. This would be used to facilitate a LRU method of eviction to maintain the most recently used N XXXStyleHelper objects.

In the short term, this may be implemented with a single XXXStyleHelper maintained (for each serialization format). And even if the above approach is taken, it may be determined that maintaining a single object is as efficient as is necessary. Or, the actual count could be overridden by the user, perhaps by adding a #define XXXStyleHelperCacheCount 5 before including the <fmtster.h> header.

[rtillery]

enhancement/19-lru is a working branch for this. The approach there uses an LRU cache design to re-use helper objects associated with style values/objects/unions.

[rtillery]

Pull request: https://github.com/HarmanFOSS/fmtster/pull/32