Modify TypedArray::create API to accept either a length or an initial slice

[jdm]

For cases when we create a new typed array with an initial slice, we always want to use the length of the provided slice. Passing the same length as another argument is redundant; we should use a custom enum argument instead (eg. CreateWith::Length(5) and CreateWith::Slice(foo)).

Code: src/typedarray.rs
Tests: cargo test --name typedarray (tests/typedarray.rs)

[simon-whitehead]

This looks like a great first PR. Mind if I give it a shot? I may need some guidance around code/test style but hopefully I won't need too much hand-holding (I can read and write Rust .. I just haven't contributed to Servo yet).

[jdm]

Please do!

[simon-whitehead]

@jdm Are you able to give me some guidance/documentation around versioning? I imagine I'll be required to bump the crate version? How does that affect how this crate version is applied to servo later? I'm just curious given I'll be changing a function signature that may have ripple effects for consumers.

[jdm]

We use a git dependency for rust-mozjs in Servo, so the crate version doesn't actually matter for that case. This crate is also not published to crates.io, so we generally have not been updating the version.

[KiChjang]

Fixed by #337.