py/parse: Add support for math module constants and float folding.

Add a new MICROPY_COMP_CONST_FLOAT feature, enabled by in mpy-cross and
when compiling with MICROPY_CONFIG_ROM_LEVEL_CORE_FEATURES.  The new
feature leverages the code of MICROPY_COMP_CONST_FOLDING to support folding
of floating point constants.

If MICROPY_COMP_MODULE_CONST is defined as well, math module constants are
made available at compile time. For example:

    _DEG_TO_GRADIANT = const(math.pi / 180)
    _INVALID_VALUE = const(math.nan)

A few corner cases had to be handled:
- The float const folding code should not fold expressions resulting into
  complex results, as the mpy parser for complex immediates has
  limitations.
- The constant generation code must distinguish between -0.0 and 0.0, which
  are different even if C consider them as ==.

This change removes previous limitations on the use of `const()`
expressions that would result in floating point number, so the test cases
of micropython/const_error have to be updated.

Additional test cases have been added to cover the new repr() code (from a
previous commit).  A few other simple test cases have been added to handle
the use of floats in `const()` expressions, but the float folding code
itself is also tested when running general float test cases, as float
expressions often get resolved at compile-time (with this change).

Signed-off-by: Yoctopuce dev <dev@yoctopuce.com>

py/mpconfig.h

@@ -490,6 +490,13 @@
 #define MICROPY_COMP_CONST (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_CORE_FEATURES)
 #endif
 
+// Whether to enable float constant folding like 1.2+3.4 (when MICROPY_COMP_CONST_FOLDING is also enabled)
+// and constant optimisation like id = const(1.2) (when MICROPY_COMP_CONST is also enabled)
+// and constant lookup like math.inf (when MICROPY_COMP_MODULE_CONST is also enabled)
+#ifndef MICROPY_COMP_CONST_FLOAT
+#define MICROPY_COMP_CONST_FLOAT (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_CORE_FEATURES)
+#endif
+
 // Whether to enable optimisation of: a, b = c, d
 // Costs 124 bytes (Thumb2)
 #ifndef MICROPY_COMP_DOUBLE_TUPLE_ASSIGN