Support compiling Happly without RTTI.

[jdumas]

I needed to compile Happly for a wasm project that wants to be compiled without RTTI, so this PR allows supporting this configuration. It comes with a custom RTTI implementation based on LLVM-style RTTI, but because they have some limitations compared to compiled-generated RTTIs (mostly when loaded across DLL boundaries), this alternative is disabled by default (and only used when RTTI are not available).