Read the GSM 03.38 message class where the spec puts it (#95)

* Regression tests for the GSM 03.38 message class behind sms.flash

* Read the GSM 03.38 message class where the spec puts it, and refuse a flash message no alphabet can carry

* Keep the code span in the message-class decision on one line

* Stop offering the flash message class as an alphabet, and check the encoding option by name

* Settle every refusable send option in one check, and record what the review left open

* Say what the flash refusal and the encoding option actually do

* Keep the README off the alphabet whose long messages do not fit

src/index.ts

@@ -4,7 +4,7 @@ export { Session } from './session.ts';
 
 export { cmds, cmdsById, commandNameById, isCommandName } from './defs/commands.ts';
 export { consts, constsById } from './defs/constants.ts';
-export { detect, encodingByDataCoding, encodings } from './defs/encodings.ts';
+export { detect, encodingByDataCoding, encodings, messageClassOf } from './defs/encodings.ts';
 export { errorNameById, errors, errorsById, isErrorName } from './defs/errors.ts';
 export { tlvs, tlvsById } from './defs/tlvs.ts';
 export { types } from './defs/types.ts';