add benchmark tests

[thaJeztah]

goos: darwin
goarch: arm64
pkg: github.com/morikuni/aec
cpu: Apple M3 Pro

BenchmarkCursor/0           1000000000     1.082 ns/op     0 B/op      0 allocs/op
BenchmarkCursor/1           19155609       68.19 ns/op     20 B/op     2 allocs/op
BenchmarkCursor/2           18512708       70.56 ns/op     20 B/op     2 allocs/op
BenchmarkCursor/20          17439208       71.64 ns/op     21 B/op     2 allocs/op
BenchmarkCursor/200         15858112       72.50 ns/op     24 B/op     2 allocs/op

BenchmarkPosition/0,0       13222344       92.66 ns/op     24 B/op     2 allocs/op
BenchmarkPosition/1,1       13894000       92.44 ns/op     24 B/op     2 allocs/op
BenchmarkPosition/2,2       12211875       94.83 ns/op     24 B/op     2 allocs/op
BenchmarkPosition/20,20     12199171       95.88 ns/op     24 B/op     2 allocs/op
BenchmarkPosition/200,200   12728629       100.7 ns/op     32 B/op     2 allocs/op
BenchmarkPosition/5,15      12093298       96.45 ns/op     24 B/op     2 allocs/op

BenchmarkEraseDisplay/0     15354889       84.39 ns/op     20 B/op     2 allocs/op
BenchmarkEraseDisplay/1     14803665       84.80 ns/op     20 B/op     2 allocs/op
BenchmarkEraseDisplay/2     14292037       81.21 ns/op     20 B/op     2 allocs/op
BenchmarkEraseDisplay/3     14696929       73.16 ns/op     20 B/op     2 allocs/op
BenchmarkEraseDisplay/20    16980390       73.61 ns/op     21 B/op     2 allocs/op
BenchmarkEraseDisplay/200   13056309       82.76 ns/op     24 B/op     2 allocs/op

BenchmarkScrollUpDown/0     1000000000     1.101 ns/op     0 B/op      0 allocs/op
BenchmarkScrollUpDown/1     18142771       68.53 ns/op     20 B/op     2 allocs/op
BenchmarkScrollUpDown/2     18104678       67.67 ns/op     20 B/op     2 allocs/op
BenchmarkScrollUpDown/20    16923913       70.32 ns/op     21 B/op     2 allocs/op
BenchmarkScrollUpDown/200   16939790       70.74 ns/op     24 B/op     2 allocs/op

BenchmarkBuilderFluent      1806829        671.0 ns/op     560 B/op    29 allocs/op
BenchmarkBuilderWith        3688992        326.4 ns/op     312 B/op    12 allocs/op

BenchmarkColor3BitF         13278399       87.57 ns/op     21 B/op     2 allocs/op
BenchmarkColor8BitF         13405442       93.59 ns/op     32 B/op     2 allocs/op
BenchmarkFullColorF         9992067        120.7 ns/op     40 B/op     2 allocs/op

PASS
ok   github.com/morikuni/aec   35.885s

[thaJeztah]

I'm working on some changes and improvements as a bit of a weekend fun project; adding some test-coverage first 😄