primitives: Add core merkle tree root calcs.

[davecgh]

This requires #2788.

This implements functions to calculate a Merkle tree root for a given set of leaves along with associated tests.

The functions are direct ports of the functions of the same names from blockchain/standalone.

Specifically, the following functions are updated and added along with their associated tests and benchmarks:

• CalcMerkleRootInPlace (equiv to standalone.CalcMerkleRootInPlace)
• CalcMerkleRoot (equiv to standalone.CalcMerkleRoot)

Note that this does not include the other functions in blockchain/standalone related to merkle root calculations from transaction trees since the primitives package does not yet implement a transaction type. Those will be implemented later after a type is introduced.

BenchmarkCalcMerkleRootInPlace
------------------------------
20_leaves      95551      12436 ns/op   0 B/op   0 allocs/op
1000_leaves     2047     591660 ns/op   0 B/op   0 allocs/op
2000_leaves     1018    1179846 ns/op   0 B/op   0 allocs/op
4000_leaves      507    2357246 ns/op   0 B/op   0 allocs/op
8000_leaves      254    4710341 ns/op   0 B/op   0 allocs/op
16000_leaves     127    9499098 ns/op   0 B/op   0 allocs/op
32000_leaves      62   18876558 ns/op   0 B/op   0 allocs/op

BenchmarkCalcMerkleRoot
-----------------------
20_leaves      92455      12840 ns/op       640 B/op   1 allocs/op
1000_leaves     1965     608127 ns/op     32769 B/op   1 allocs/op
2000_leaves      969    1261865 ns/op     65536 B/op   1 allocs/op
4000_leaves      493    2439323 ns/op    131073 B/op   1 allocs/op
8000_leaves      246    4850361 ns/op    262144 B/op   1 allocs/op
16000_leaves     122    9826700 ns/op    516101 B/op   1 allocs/op
32000_leaves      63   19420881 ns/op   1024008 B/op   1 allocs/op

This is work towards #2786.

[JoeGruffins]

benchmarks

goos: linux
goarch: amd64
pkg: github.com/decred/dcrd/internal/staging/primitives
cpu: AMD Ryzen 9 3900XT 12-Core Processor           
BenchmarkCalcMerkleRootInPlace/20_leaves-24               140908              9020 ns/op               0 B/op          0 allocs/op
BenchmarkCalcMerkleRootInPlace/1000_leaves-24               2755            409990 ns/op               0 B/op          0 allocs/op
BenchmarkCalcMerkleRootInPlace/2000_leaves-24               1438            869987 ns/op               0 B/op          0 allocs/op
BenchmarkCalcMerkleRootInPlace/4000_leaves-24                741           1642815 ns/op               0 B/op          0 allocs/op
BenchmarkCalcMerkleRootInPlace/8000_leaves-24                338           3413965 ns/op               0 B/op          0 allocs/op
BenchmarkCalcMerkleRootInPlace/16000_leaves-24               177           6479413 ns/op               0 B/op          0 allocs/op
BenchmarkCalcMerkleRootInPlace/32000_leaves-24                91          13891543 ns/op               0 B/op          0 allocs/op
BenchmarkCalcMerkleRoot/20_leaves-24                       94971             15375 ns/op             640 B/op          1 allocs/op
BenchmarkCalcMerkleRoot/1000_leaves-24                      1644            718782 ns/op           32768 B/op          1 allocs/op
BenchmarkCalcMerkleRoot/2000_leaves-24                       776           1392608 ns/op           65538 B/op          1 allocs/op
BenchmarkCalcMerkleRoot/4000_leaves-24                       410           2634010 ns/op          131072 B/op          1 allocs/op
BenchmarkCalcMerkleRoot/8000_leaves-24                       261           5422578 ns/op          262144 B/op          1 allocs/op
BenchmarkCalcMerkleRoot/16000_leaves-24                      120           9022608 ns/op          516096 B/op          1 allocs/op
BenchmarkCalcMerkleRoot/32000_leaves-24                       62          17837654 ns/op         1024004 B/op          1 allocs/op