sysfs: improve SPI heap utilization and performance
sysfs: improve SPI heap utilization and performance

- Put spiConn inside the SPI object instead of allocating it on the heap.
- Create a 4 items array of spiIOCTransfer inside the SPI object to remove
  memory allocation when doing transfers of 4 items or less.

This results in dramatic improvement when using spi.Conn.Tx().

Test environment:
- go 1.10.2
- Intel(R) Xeon(R) CPU E5-2690 0 @ 2.90GHz
- Raspberry Pi 3

Intel: 60% faster
Before: BenchmarkSPI    10000000      167 ns/op     32 B/op     1 allocs/op
After:  BenchmarkSPI    20000000     68.0 ns/op      0 B/op     0 allocs/op

RPi3: 37% faster
Before: BenchmarkSPI     1000000     1795 ns/op     32 B/op     1 allocs/op
After:  BenchmarkSPI     1000000     1125 ns/op      0 B/op     0 allocs/op

google/periph