libsolutil: append SHA-256 input via vector::insert

picosha2::hash256_one_by_one::process appends its input range to the
internal buffer with std::copy + std::back_inserter, producing O(N)
individual push_back calls (one capacity check per byte). For the
random-access iterators this template is used with, vector::insert at
the end is equivalent and does a single resize + bulk copy.

Functional behavior is unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: msooseth

Changelog.md

@@ -3,6 +3,7 @@
 Language Features:
 
 Compiler Features:
+* General: Speed up SHA-256 hashing (`picosha2`) by avoiding per-byte `vector::push_back` when consuming input.
 
 Bugfixes:
 

libsolutil/picosha2.h

@@ -193,7 +193,7 @@ class hash256_one_by_one {
     template <typename RaIter>
     void process(RaIter first, RaIter last) {
         add_to_data_length(static_cast<word_t>(std::distance(first, last)));
-        std::copy(first, last, std::back_inserter(buffer_));
+        buffer_.insert(buffer_.end(), first, last);
         std::size_t i = 0;
         for (; i + 64 <= buffer_.size(); i += 64) {
             detail::hash256_block(h_, buffer_.begin() + long(i),