Multipart parser and other fixes

• Introduces support for parsing multipart POSTs in an efficient and simple way. Runs about 155x faster than Node.js alternatives - especially on parts larger than a few hundred kilobytes.
• The library now compiles without any warnings using -Wall -Wextra -Wpedantic -Wsign-conversion -Wconversion on both GCC and Clang. Hundreds of (non-default) warnings have been fixed in this release and from now we aim to keep things pedantically clean.
• Fixes a permessage-deflate regression introduced in v0.17.4.
• Fixes parsing bugs when compiling using WITH_PROXY=1 (introduced in v18.1.0).