* add type for Num.toStr
* create new lowlevel
* delete types and Symbol for fromInt and fromFloat
* leave LowLevel::{StrFromFloat,StrFromInt}
* match on LowLevel::NumToStr and figure out the layout to decide
which build function to delegate to
When I created this (at the very beginning of the Wasm backend),
I didn't really have a clear reason for it. I just thought it might end up
making sense treat heap pointers differently from numbers, somehow.
But the semantic differences between pointers and other numbers is not relevant
to WasmLayout. The semantics are clear from where the Symbol appears in the IR.
Also we were storing heap pointers in locals, for no real reason.
And the fact that it's *different* meant a lot of new cases in match expressions,
to do the exact same thing as Primitives but with a pointless difference.
Until now, we haven't really used this variant in any of our tests.
But the refcount pointer needed it... and everything broke!
