Checks that exported symbols are valid C identifiers. Refs #800.

Examples of error messages:
```
read.nim(5, 21) Error: invalid exported symbol, can't be zero length
read.nim(5, 21) Error: invalid exported symbol, '0penpeitor' can't start with a number
read.nim(5, 21) Error: invalid exported symbol, 'peñpeitor' contains bad character at byte 2
```