Add alloc feature to support no_std without an allocator

[fbrozovic]

Gate the Vec-returning block APIs behind a new alloc feature, enabled by default via std. Without it only the _into variants are available; compression places its hash table on the stack, or the table can be provided via compress_into_with_table, with const constructors so it can be stored in a static.

Fixes: #220

[PSeitz]

What happens if there is no allocator available? Will it fail to compile or a runtime error?

[fbrozovic]

The existing behavior is unchanged - the crate fails to compile if there is no allocator available and the alloc feature is enabled:

error: no global memory allocator found but one is required; link to std or add `#[global_allocator]` to a static item that implements the GlobalAlloc trait