Demangle function names in backtraces

[jclark]

When a panic prints out a function name, it should demangle it.

Mangling is described here: #448 (comment)

[jclark]

The backtrace should make sense to a Ballerina programmer. It should only include stack frames corresponding to Ballerina functions. These all start with _B. It needs to reverse the mangling that is done in nback/symbol.bal and turn it into something that makes sense in Ballerina.

Non-public names should be printed without any prefix.

For a public name e.g. with module id ballerina/lang.array and local name push, I suggest we print it as lang.array:push i.e. leave out the organization name.

The C runtime should implement a function _bal_print_mangled_name(const char *, FILE *)