path: replace ImmutablePath interface with struct

[Jorropo]

Let's not repeat ipfs/go-block-format#45 interface for struct with one implementation and no value added.

[Jorropo]

Skip changelog since this is not a breaking change yet (we havn't released path yet).

[codecov]

Codecov Report

Merging #481 (7e95df2) into main (85c180e) will increase coverage by 0.03%.
The diff coverage is 42.42%.

@@            Coverage Diff             @@
##             main     #481      +/-   ##
==========================================
+ Coverage   65.73%   65.76%   +0.03%     
==========================================
  Files         207      207              
  Lines       25131    25131              
==========================================
+ Hits        16520    16528       +8     
+ Misses       7149     7143       -6     
+ Partials     1462     1460       -2     

Files	Coverage Δ
path/path.go	96.80% <90.00%> (ø)
gateway/handler.go	77.89% <50.00%> (ø)
gateway/blocks_backend.go	43.00% <27.27%> (ø)
gateway/handler_unixfs__redirects.go	48.54% <0.00%> (ø)

... and 11 files with indirect coverage changes

[hacdias]

I think this change looks sensible. I'm almost sure this was part of the initial PR for the path but I changed it somewhere. Just want to note that Path should stay an interface, and namespaces should be a string. This allows for more experimentation from other people.

But ImmutablePath does indeed make sense to be a struct.