fix: refactor taggedlen/rawsmallvec and visibility

[alejandro-vaz]

this PR refactors TaggedLen and RawSmallVec exclusively to address visibility issues

• moves the RawSmallVec implementation into rawsmallvec.rs
• moves TaggedLen and its implementations to taggedlen.rs
• makes RawSmallVec implementation methods public
• allows TaggedLen to be exported under the internals feature

RawSmallVec methods are made public so they can be seen when exported under the internals feature to downstream crates

consequently, TaggedLen needs to be made public in order to allow the try_grow_raw method in RawSmallVec to be public and callable

note: borsh.rs has been changed in order not to use the first field, which was incorrect. using the first field also meant that it had to be made public but it is kept private. this is also implemented in #506 but I had to port it here