[memprof] Add CallStackRadixTreeBuilder (#93784)

Call stacks are a huge portion of the MemProf profile, taking up 70+%
of the profile file size.

This patch implements a radix tree to compress call stacks, which are
known to have long common prefixes.  Specifically,
CallStackRadixTreeBuilder, introduced in this patch, takes call stacks
in the MemProf profile, sorts them in the dictionary order to maximize
the common prefix between adjacent call stacks, and then encodes a
radix tree into a single array that is ready for serialization.

The resulting radix array is essentially a concatenation of call stack
arrays, each encoded with its length followed by the payload, except
that these arrays contain "instructions" like "skip 7 elements
forward" to borrow common prefixes from other call stacks.

This patch does not integrate with the MemProf
serialization/deserialization infrastructure yet.  Once integrated,
the radix tree is expected to roughly halve the file size of the
MemProf profile.