Optimize include_bin! for large inputs

Previously an ExprLit was created *per byte* causing a huge increase in memory
bloat. This adds a new `lit_binary` to contain a literal of binary data, which
is currently only used by the include_bin! syntax extension. This massively
speeds up compilation times of the shootout-k-nucleotide-pipes test

    before:
        time: 469s
        memory: 6GB
        assertion failure in LLVM (section too large)

    after:
        time: 2.50s
        memory: 124MB

Closes #2598
auto merge of #9851 : alexcrichton/rust/include_bin, r=huonw

Previously an ExprLit was created *per byte* causing a huge increase in memory
bloat. This adds a new `lit_binary` to contain a literal of binary data, which
is currently only used by the include_bin! syntax extension. This massively
speeds up compilation times of the shootout-k-nucleotide-pipes test

    before:
        time: 469s
        memory: 6GB
        assertion failure in LLVM (section too large)

    after:
        time: 2.50s
        memory: 124MB

Closes #2598