Reduce code bloat from managed allocations

In commit d7f5e43 "core::rt: Add the local heap to newsched tasks",
local_malloc and local_free have become rather big and their forced
inlining causes quite a bit of code bloat. Compile times for crates
affected by the bloat (e.g. rustc) improve, while others (e.g. libstd)
seem to be unaffected, so I guess the inlining doesn't gain us much.

Sizes:
```
               | librustc   | libsytax
---------------|–-----------|------------
with inlining  | 18,547,824 |  7,110,848
w/o inlining   | 15,092,040 |  5,518,608