Building for wasm doesn't work (well, building works but running does not)

[apoelstra]

Related to #119

We currently depend on the following symbols:

• malloc
• free

(We additionally depended on abort and printf until #115 )

This prevents use with rust-wasm for this library and all downstream users (e.g. rust-bitcoin). While doing any cryptography in a browser is very ill-advised, rust-secp does a lot more than just signing and verification (it can also parse and display keys), and its downstream deps such as rust-bitcoin do many things that certainly should be possible and easy to do with rust-wasm.

We also don't want to encourage users to find other pure-Rust secp256k1 libraries with dramatically less review and QA on them, and to use these in a Bitcoin context.

See rust-bitcoin/rust-bitcoin#240 for earlier discussion.

[apoelstra]

I was able to get a wasm application working which parses and Javascript alerts a public key. Unfortunately to do so I needed to

1. Hack Implementing pre allocation context creation #116 to eliminate all calls to context_create and context_destroy (this I expected).
2. Actually delete context_create, scratch_space_create, context_destroy, scratch_space_destroy from the libsecp source. Deleting the FFI bindings was not enough (this I did not expect).

[elichai]

Yeah, we need to eliminate the symbols, we don't use the scratch space, right?

[apoelstra]

Correct, we do not use the scratch space. (It will eventually be used for MuSig but before then we'll figure out a preallocated solution for it.)