Warn if Type::string size is not matched when assignment
Warn if Type::string size is not matched when assignment

For example:

```
% sudo ./src/bpftrace -e 'tracepoint:block:block_rq_issue { @ = args->rwbs; @ = "foo";}'
stdin:1:53-54: WARNING: String size mismatch: 8 != 64. The value may be truncated.
tracepoint:6b41b4b0cf195b3026d36ba7cf6c312ab60f19cc@ = "fooooooo";}
                                                    ~
```

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace