Share data memory of String terms

[mkaput]

Currently, String terms own their data. Some read-only operations such as slicing make unnecessary copies of data.

This can be optimised by storing data in a separate entity, called Buffer, where Strings themselves only store the pointer to the Buffer and offsets in its data. Buffers could be allocated with a custom allocator for faster allocations, and to introduce some interning for memory usage reduction.

Strings could be further optimised by introducing Copy-on-Write semantics, but this requires investigation whether this would provide any performance gains.

Tasks (each should be a separate PR):

• Introduce the concept of Buffers and optimise slices
• Implement custom memory area for Buffers and intern small buffers
• Investigate Copy-on-Write semantics for Buffers, and implement it would provide decent improvements. Optimize string "mutation" (replacing sub-ranges etc.)