feat: add tr_strbuf (#2810)

* feat: add tr_strbuf class for building tmp strings

Based on fmt::basic_memory_buf, this is a growable string buffer that
has an initial size that's large enough to build most filenames or URLs
without needing heap allocations.

Adds a couple of extra helpers such as a `c_str()` method to make dealing
with old zero-terminated string APIs easier.

daemon/CMakeLists.txt

@@ -13,11 +13,6 @@ include_directories(
     SYSTEM
     ${CURL_INCLUDE_DIRS}
     ${EVENT2_INCLUDE_DIRS}
-    ${LIBFMT_INCLUDE_DIRS}
-)
-
-add_definitions(
-    ${LIBFMT_DEFINITIONS}
 )
 
 set(${PROJECT_NAME}_SOURCES