Make core::StringBuffer non-template

[gavv]

core::StringBuffer implements simple buffer for storing strings.

Currently it's a template class. It has only one template parameter: EmbeddedCapacity, which defines number of bytes embedded directly into StringBuffer object. (Short strings can be stored in this capacity without heap allocations).

However, we can still cover most use cases if we hard-code specific value of EmbeddedCapacity instead of making it configurable. For example, we can set it to 32 bytes.

If we do so, we can make StringBuffer non-template. Having lots of templates increases code size and compilation times. Also, it will allow us to simplify implementation of core::StringBuilder.

So the steps are simple:

• hard-code EmbeddedCapacity to 32
• make StringBuffer non-template
• move implementations of methods to .cpp file

Then cleanup StringBuilder:

• make DynamicBufferWriter non-template
• move implementations of methods of DynamicBufferWriter to .cpp file
• move implementations of StringBuilder ctor to .cpp file

[ForeverASilver]

@gavv I'll take a crack at it if you assign it to me

[gavv]

Thanks!