Fix CSP error for wasm bindgen

[evq]

Fixes #91

Access the global object via self directly instead of the current return this function created from a string. Creating a function from a string results in a CSP error as it requires unsafe-eval.

See tweetnacl which uses this strategy (although it also checks for msCrypto for IE11 compatibility - here we don't because IE11 does not have wasm support).

Tested on node, firefox and brave (chromium) using the wasm-pack test.

[newpavlov]

Thank you! LGTM.

@josephlr
If you don't have objections I will merge it.

[josephlr]

I think we would be better off using js_sys to get the global object, as that seems to be the more supported way of doing things

[evq]

The js_sys strategy is a bit overkill here since we only need a way to fetch the global object on browsers, for which self should always work. They have to go through a lot of extra hoops because they're trying to make it possible to get the global object even on nodejs and other non-browser platforms.

[josephlr]

@evq that makes sense. @newpavlov we should think about using js_sys/web_sys in a different PR, but I think merging this and fixing #91 before releasing 0.1.10 is a good idea.

[newpavlov]

I think we can discuss it in #63, but I still don't see a reason why we would want to use crates built on top of wasm-bindgen instead of using it directly.