Add @Spell clusters to Vim syntax file

Add @Spell clusters to Vim syntax highlighting file to do spell checking
only inside comments and strings
added empty() to Peekable
Introduced a chapter that describes how to perform callbacks from C to Rust
Further details on channel idea
Support linking of rlib files in rustpkg
Fix indention
Update guide-ffi.md

Simplified the first examples to demonstrate callbacks without other threads involved and shortened the elaboration about async callbacks.
extra::ebml: Make reader::Res public

Since reader::vuint_at() returns a result of type reader::Res it makes sense
to make it public.

Due to rust's current behavior of externally referenced private structures,
https://github.com/mozilla/rust/issues/10573, you could still use the result and
assign it to a variable if you let the compiler do the type assignment,
but you could not explicitly annotate a variable to hold a reader::Res.
extra::ebml: Add unit test for vuint_at()
ebml::extra: Optimize reader::vuint_at()

Use a lookup table, SHIFT_MASK_TABLE, that for every possible four
bit prefix holds the number of times the value should be right shifted and what
the right shifted value should be masked with. This way we can get rid of the
branches which in my testing gives approximately a 2x speedup.
renamed empty() to is_empty()
fixup! ebml::extra: Optimize reader::vuint_at()
Issue #3511 - Rationalize temporary lifetimes.

Major changes:

- Define temporary scopes in a syntax-based way that basically defaults
  to the innermost statement or conditional block, except for in
  a `let` initializer, where we default to the innermost block. Rules
  are documented in the code, but not in the manual (yet).
  See new test run-pass/cleanup-value-scopes.rs for examples.
- Refactors Datum to better define cleanup roles.
- Refactor cleanup scopes to not be tied to basic blocks, permitting
  us to have a very large number of scopes (one per AST node).
- Introduce nascent documentation in trans/doc.rs covering datums and
  cleanup in a more comprehensive way.
Use as_slice() method on option
Remove FIXMEs and add license
Cleanup trait callees
doc: Boxes diagram shows enum value inside box.
doc: add note below diagram about memory layout.
Remove typo
Update test run summary