🚧 manual lexer

This commit removed the re2c lexer and replaced it by a manual version.
Its integration is not yet complete: number parsing does not respect
locales or overflows. Furthermore, parsing does not need to end with
EOF. Therefore, a lot of test cases fail. The idea is to push this
branch forward so we can conduct performance comparisons. So far, a
nice side effect are better diagnosis messages in case of parse errors.

Makefile

@@ -1,9 +1,5 @@
 .PHONY: pretty clean ChangeLog.md
 
-# used programs
-RE2C := $(shell command -v re2c 2> /dev/null)
-SED = sed
-
 # main target
 all:
 	$(MAKE) -C test
@@ -183,13 +179,6 @@ clang_sanitize: clean
 # maintainer targets
 ##########################################################################
 
-# create scanner with re2c
-re2c: src/json.hpp.re2c
-ifndef RE2C
-	$(error "re2c is not available, please install re2c")
-endif
-	$(RE2C) -W --utf-8 --encoding-policy fail --bit-vectors --nested-ifs --no-debug-info $< | $(SED) '1d' > src/json.hpp
-
 # pretty printer
 pretty:
 	astyle --style=allman --indent=spaces=4 --indent-modifiers \
@@ -197,7 +186,7 @@ pretty:
 	   --indent-col1-comments --pad-oper --pad-header --align-pointer=type \
 	   --align-reference=type --add-brackets --convert-tabs --close-templates \
 	   --lineend=linux --preserve-date --suffix=none --formatted \
-	   src/json.hpp src/json.hpp.re2c test/src/*.cpp \
+	   src/json.hpp test/src/*.cpp \
 	   benchmarks/benchmarks.cpp doc/examples/*.cpp