[yul-phaser] Make Program and Population classes accept source code rather than file path

- I need some sample .yul files for testing but I see that existing tests generally have source code hard-coded in them rather than in standalone .yul files. There are lots of .yul files but they seem to be automatically processed by a special test case rather loaded ad-hoc by manually created tests.
- Program and Population required a file name until now. I'm making them accept loaded source code to be able to give them data hard-coded in a test.

tools/yulPhaser/Program.cpp

@@ -38,11 +38,8 @@
 #include <libyul/optimiser/OptimiserStep.h>
 #include <libyul/optimiser/Suite.h>
 
-#include <libsolutil/CommonIO.h>
 #include <libsolutil/JSON.h>
 
-#include <boost/filesystem.hpp>
-
 #include <cassert>
 #include <memory>
 
@@ -60,10 +57,11 @@ ostream& operator<<(ostream& _stream, Program const& _program);
 
 }
 
-Program Program::load(string const& _sourcePath)
+Program Program::load(CharStream& _sourceCode)
 {
+	// ASSUMPTION: parseSource() rewinds the stream on its own
 	Dialect const& dialect = EVMDialect::strictAssemblyForEVMObjects(EVMVersion{});
-	unique_ptr<Block> ast = parseSource(dialect, loadSource(_sourcePath));
+	unique_ptr<Block> ast = parseSource(dialect, _sourceCode);
 	unique_ptr<AsmAnalysisInfo> analysisInfo = analyzeAST(dialect, *ast);
 
 	Program program(
@@ -95,14 +93,6 @@ string Program::toJson() const
 	return jsonPrettyPrint(serializedAst);
 }
 
-CharStream Program::loadSource(string const& _sourcePath)
-{
-	assertThrow(boost::filesystem::exists(_sourcePath), InvalidProgram, "Source file does not exist");
-
-	string sourceCode = readFileAsString(_sourcePath);
-	return CharStream(sourceCode, _sourcePath);
-}
-
 unique_ptr<Block> Program::parseSource(Dialect const& _dialect, CharStream _source)
 {
 	ErrorList errors;