A Path contains a boxed slice of PathCommand, which is the
shrunk-to-fit Vec<PathCommand> from the PathBuilder.  The boxed slice
doesn't have the overhead of the capacity value.

For the huge map file in issue #574, this reduces path overhead by
a nice bit:

Before:
--------------------------------------------------------------------------------
  n        time(i)         total(B)   useful-heap(B) extra-heap(B)    stacks(B)
--------------------------------------------------------------------------------
 23 22,751,613,855    1,560,916,408    1,493,746,540    67,169,868            0
95.70% (1,493,746,540B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc.
->43.94% (685,929,024B) 0x49FF0BB: alloc (alloc.rs:84)
| ->43.94% (685,929,024B) 0x49FF0BB: exchange_malloc (alloc.rs:206)
|   ->43.94% (685,929,024B) 0x49FF0BB: new<rsvg_internals::element::Element> (boxed.rs:121)
|   ...
|
->36.05% (562,764,384B) 0x49CC457: realloc (alloc.rs:128)
| ->36.05% (562,764,384B) 0x49CC457: realloc (alloc.rs:187)
|   ->36.05% (562,764,384B) 0x49CC457: reserve_internal<rsvg_internals::path_builder::PathCommand,alloc::alloc::Global> (raw_vec.rs:693)
|     ->36.05% (562,764,384B) 0x49CC457: alloc::raw_vec::RawVec<T,A>::reserve (raw_vec.rs:520)
|       ->22.36% (349,090,560B) 0x49927B0: reserve<rsvg_internals::path_builder::PathCommand> (vec.rs:501)
|       | ->22.36% (349,090,560B) 0x49927B0: push<rsvg_internals::path_builder::PathCommand> (vec.rs:1150)

After:
--------------------------------------------------------------------------------
  n        time(i)         total(B)   useful-heap(B) extra-heap(B)    stacks(B)
--------------------------------------------------------------------------------
 30 22,796,106,012    1,553,581,072    1,329,943,324   223,637,748            0
85.61% (1,329,943,324B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc.
->44.15% (685,929,024B) 0x49FF25B: alloc (alloc.rs:84)
| ->44.15% (685,929,024B) 0x49FF25B: exchange_malloc (alloc.rs:206)
|   ->44.15% (685,929,024B) 0x49FF25B: new<rsvg_internals::element::Element> (boxed.rs:121)
|
The ones with slack space:
->22.53% (350,014,176B) 0x4A23300: realloc (alloc.rs:128)
| ->22.53% (350,014,176B) 0x4A23300: realloc (alloc.rs:187)
|   ->22.53% (350,014,176B) 0x4A23300: shrink_to_fit<rsvg_internals::path_builder::PathCommand,alloc::alloc::Global> (raw_vec.rs:633)
|     ->22.53% (350,014,176B) 0x4A23300: shrink_to_fit<rsvg_internals::path_builder::PathCommand> (vec.rs:623)
|       ->22.53% (350,014,176B) 0x4A23300: alloc::vec::Vec<T>::into_boxed_slice (vec.rs:679)
|         ->22.53% (350,014,176B) 0x4A03410: into_path (path_builder.rs:316)
|
| ...
|
The ones without slack space:
->03.48% (54,029,952B) 0x49CC577: realloc (alloc.rs:128)
| ->03.48% (54,029,952B) 0x49CC577: realloc (alloc.rs:187)
|   ->03.48% (54,029,952B) 0x49CC577: reserve_internal<rsvg_internals::path_builder::PathCommand,alloc::alloc::Global> (raw_vec.rs:693)
|     ->03.48% (54,029,952B) 0x49CC577: alloc::raw_vec::RawVec<T,A>::reserve (raw_vec.rs:520)
|       ->02.98% (46,292,544B) 0x49927E0: reserve<rsvg_internals::path_builder::PathCommand> (vec.rs:501)
|       | ->02.98% (46,292,544B) 0x49927E0: push<rsvg_internals::path_builder::PathCommand> (vec.rs:1150)
|       |   ->02.98% (46,292,544B) 0x49927E0: line_to (path_builder.rs:325)