Implement preprocess_use_blocks

While being very similar in style to the C version, it takes 27 lines
of code less to implement it due to the niceties of C++ (41 lines vs
68).

asm/preprocesser.cpp

@@ -11,3 +11,55 @@
  */
 
 #include "./preprocesser.hpp"
+#include "./base.hpp"
+
+#include <algorithm>
+#include <unordered_map>
+
+using std::pair, std::vector, std::make_pair, std::string, std::string_view;
+
+#define ERR(E) std::make_pair(tokens, (E))
+#define VAL(E) std::make_pair(E, pp_err_t{pp_err_type_t::OK})
+
+pair<vector<token_t *>, pp_err_t>
+preprocess_use_blocks(vector<token_t *> tokens)
+{
+  vector<token_t *> new_tokens;
+  for (size_t i = 0; i < tokens.size(); ++i)
+  {
+    token_t *t = tokens[i];
+    if (t->type == token_type_t::PP_USE)
+    {
+      if (i + 1 >= tokens.size() ||
+          tokens[i + 1]->type != token_type_t::LITERAL_STRING)
+      {
+        new_tokens.clear();
+        return ERR(pp_err_t(pp_err_type_t::EXPECTED_STRING, t));
+      }
+
+      token_t *name = tokens[i + 1];
+      auto source   = read_file(name->content.c_str());
+      if (!source)
+      {
+        new_tokens.clear();
+        return ERR(pp_err_t(pp_err_type_t::FILE_NONEXISTENT, name));
+      }
+
+      std::vector<token_t *> ftokens;
+      lerr_t lerr = tokenise_buffer(source.value(), ftokens);
+      if (lerr != lerr_t::OK)
+      {
+        new_tokens.clear();
+        return ERR(pp_err_t(pp_err_type_t::FILE_PARSE_ERROR, name, lerr));
+      }
+
+      new_tokens.insert(new_tokens.end(), ftokens.begin(), ftokens.end());
+
+      i += 2;
+    }
+    else
+      new_tokens.push_back(new token_t{*t});
+  }
+  return VAL(new_tokens);
+}
+