sei() not called as it should have been several times

[eclab]

cli() (aka "noInterrupts()") is called multiple times in the NeoSWSerial code, but only once is sei() (aka "interrupts()") called to reenable the interrupts. This appears to be a significant error in a number of places.

Now I'm not sure if this is necessary, for example, in the ignore() function where only cli() is called. However in the write() function, a missing sei() is definitely messing with accuracy of written code, particularly after patching with the long-standing pull-request to properly handle writes from multiple serial ports (#48).

[eclab]

I think it's got SREG being set each time, so I'm closing this.